PART I.—MORPHOLOGY AND PHYSIOLOGY (continued 5)

Fig. 212.—Hairs of Datana: f, formative hair-cell; c, cuticula; p, pigmented layer; hy, hypodermis.

The integument of many insects contains fine canals passing through the chitinous layers and opening externally in minute pores. Certain of the pore-canals communicate with hollow setæ which sit directly over the pores; other pores form the external openings of dermal glands, but in many cases they are empty or only filled with air, and do not have any hairs connected with them. Each of these pores communicates with a hair-forming hypodermal cell, called by Graber a trichogen.

Setæ (“hairs” and bristles).—The setæ of insects are, as in worms, processes of the cuticle originating from certain of the hypodermal cells. They arise either from a ring-like pit, or from a minute tubercle, and are usually situated at the outlet of a pore-canal, which connects with an underlying cell of the hypodermis (Fig. 212). They are, then, bristle or hair-like processes arising from the hypodermis. Where the hairs or setæ are rubbed off, their site is indicated by a minute ring like a follicle in the 189chitinous integument. The cuticular hair, says Leydig, is in its first condition the secretion of the cellular element of the skin, and a thread-like continuation of the cell-body may rise up through the pore-canal into the centre of the hair, remaining there permanently.

While the setæ are usually simple, they are often branched, plumose, or spinulose, as in larval Hemerobiidæ, Anthrenus, and Dermestes, the larvæ of certain coccinellid beetles, notably Epilachna, and of Cassida, the larvæ of arctians, etc., and in bees (Anthophila, Megachile, Osmia, Colletes, Apis, etc.).

The use of these spinulose, plumose, and twisted hairs in the bees is clearly shown by J. B. Smith, who states that as these insects walk over flowers, the pollen grains adhere to the vestiture, “and this also accounts for the fact, probably noticed by every observant fruit-grower, that bees frequently bury themselves completely in the blossoms, or roll over every part of them. Such insects are after pollen, not honey, and by so rolling about, the pollen grains are brought into contact with and adhere to the surface of the insect.” The syrphid flies also pollenize flowers, the pollenizing of chrysanthemums being effected, as Smith states, by Eristalis tenax, and he adds that the body vestiture of the syrphids “is often composed of spurred and branched hairs.” (For reference to gathering hairs, see p. 45.)

Fig. 213.—Cuticular spinules of larva of Adoneta: a, b, c, d, different forms; e, e′, caltrops.

Certain remarkable spines occur in limacodid larvæ, notably Empretia and Adoneta. These we have called caltrops spines, from their resemblance to the caltrops formerly used in repelling the attacks of cavalry. They are largely concerned in producing the poisonous and irritating effects resulting from contact with the caterpillars of these moths, and are situated in scattered groups near the end of the tubercles. A group of three is represented at Fig. 213, e. They are not firmly embedded in the cuticle, but on the contrary 190appear to become very easily loosened and detached, and they probably, when brought into contact with the skin of any aggressor, burrow underneath, and are probably in part the cause of the continual itching and annoyance occasioned by these creatures. It will be seen by reference to Fig. 213, e′, that the body of the spine is spherical, with one large, elongated, conical spine arising from it, the spherical base being beset with a number of minute, somewhat obtuse spinules.

Fig. 214.—Glandular hairs of caterpillars. A, Dasylophia anguina: a, of body; b, of head; c, of prothoracic shield. B, Ceratosia tricolor: a, on body; b, on abdominal legs. C, Schizura ipomeæ: a, from third thoracic segment; b, from larva stage II; c, simple setæ from minute warts.

Glandular hairs and spines.—In some insects occur fine, minute, hollow setæ from which exude, perhaps through pore-canals of extreme fineness, droplets of a clear watery or plasma-like sticky fluid. The club-shaped tenent hairs of the feet of Collembola, and the hairs fringing the feet of Diptera, are modified glandular hairs. Here they serve to give out a sticky fluid enabling the insect to walk on smooth surfaces; they end in a vesicle-like bulbous expansion, which may contain numerous pore-canals. Those of caterpillars were first noticed by Zeller, and Dimmock has particularly described those of 191the larvæ of Pterophoridæ. They are either club-shaped, or variously forked at the end (Fig. 214, B, a). They are usually replaced after the first larval moult by ordinary, simple, solid, pointed setæ, and their use in caterpillars is as yet unknown. Whether these hairs, as seems most probable, arise from a specialized glandular hypodermal cell, or not, has not yet been discovered.

Fig. 215.—A, group of setæ arising from a subdorsal tubercle: cut, the cuticle; hy, the hypodermis; sc, the enlarged and specialized cells of the hypodermis which secrete the spines themselves; pglc, the nuclei which secrete the venomous fluid which fills the cavity of the seta (s), seen at p in a broken spine. B, a short entire, and a long broken seta (s-p); pgle, four poison cells; p, the poison in the hollow of the spine.

These temporary fine glandular hairs are probably the homologues of the larger true glandular bristles and spines of the later stages of certain lepidopterous larvæ, which are brightly colored and lead an exposed life, living through a large part of the summer. In these structures the bristles or spines are hollow, filled with a poisonous secretion formed in a single large, or several smaller specialized hypodermal cells situated under the base of the spine. In the venomous spines of Lagoa crispata the poisonous fluid in the larger spines (Figs. 215, C, 216, b) is secreted in several large cells situated at the base of the spine, and this is the usual form. In the finer spines of a large tubercle (Figs. 215, A, 216) there appears to be a differentiation of the hypodermal cells into two kinds, the large, basal deep-seated, setigenous cells (216, sc) and the poison-secreting nuclei (216, pglc) situated nearer the base of the setæ. The spines being filled with poison and breaking into bits in the skin of the hands or neck, cause great irritation and smarting. These nettling or poisonous hairs or spines are especially venomous in the larva of 192Orgyia, Empretia stimulea, Hyperchiria io, the larvæ of the saturnians (Fig. 217) and lasiocampids, etc. They rarely occur in insects of other orders, though the skin of Telephorus is said by Leydig to bear glandular hairs.

Fig. 216.—Section of a subdorsal tubercle from a larva in stage 1: sc, the setigenous cells, one for each seta; pglc, nuclei by which the poison is secreted; s, seta; p, poison in middle of a broken spine; cut, cuticle; sd, tub, spinulated surface of the subdorsal tubercle.

Leydig states that in the stout bristles of Saturnia there is, as in the integument of the body, a homogeneous cuticula, under which is the cellular matrix (hypodermis), and the clear contents (hyaloplasma) are secreted from the blood. The cell-structure of the hairs consist, as in the cells of the body, of spongioplasma and hyaloplasma. Leydig has observed the droplets of the secretion of the caterpillar of Saturnia carpini oozing through distinctly observable pores, and states that there are similar openings in the hairs and scales. Dewitz found easily observable openings at the end of the hair of a large exotic weevil (Fig. 130).

The advanced nymph of Psylla is also armed with clavate glandular hairs (Fig. 178).

Fig. 217.—Armature of last four segments of Callosamia promethea: a, a dorsal seta; b, one showing the poison (p) within.

The tubercles are outgrowths of the body-walls; they are either smooth, warty, or spiny, as in many caterpillars. While the armature of insects is of little morphological 193importance, it is evidently of great biological importance, the welfare or even the life of the insect depending upon it; and it varies in each species of insect, especially in Diptera, where the position of even a single seta characterizes the species.

Fig. 218.—Section through an antennal pectination of Saturnia carpini: a, hypodermis, formative cells of the hairs (c); d, cuticula; e, trachea.—After Semper.

Fig. 219.—Flattened hairs from the lateral tufts of larva of Gastropacha americana: A, three from the lateral tuft of Heteropacha rileyana.

The mode of development of the hairs was first described by Semper. In the pectination of the antenna of Saturnia carpini he observed that the hairs arise, like the scales of the wings, from large round formative-cells lying in the cavity, which send out through the hypodermis and cuticle a long slender process which finally becomes the hair (Fig. 218).

Tactile hairs are those setæ arising over nerve cells or nerve terminations and will be discussed under the organs of sense.

Fig. 220.—The same in G. quercifolia: a, a small hair ending in two minute processes.

Scales.—In very rare cases the hairs of caterpillars (Fig. 219) are flattened and scale-like, and this passage in the same insect of cylindrical hairs into flattened scale-like ones, shows that the scales are only modified hairs. Also, as we shall see farther on, Semper has proved that their mode of origin is identical. While true scales are characteristic of Synaptera (Thysanura and Colembola), as well as Lepidoptera and Trichoptera, they also occur in the Psocidæ (Amphientomum), in 194many Coleoptera (Curculionidæ, Cleridæ, Ptinidæ, Dermestidæ, Byrrhidæ, Scarabæidæ, Elateridæ, and Cerambycidæ), and in the Culicidæ, and a few other Diptera, though they are especially characteristic of the Lepidoptera, not a species of this great order being known to be entirely destitute of them.

Fig. 221.—Flattened and spinulated hairs of tufts of larva of Acronycta hastulifera.

Fig. 222.—Scales from dorsal tuft, on second thoracic segment of larva of Gastropacha quercifolia.

The scales vary much in shape, but are more or less tile-like, attached to the surface of the body or wing by a short slender pedicel, and are more loosely connected with the integument than the hairs, which are thicker at the base or insertion than beyond.

The markings of the scales, both of Synaptera and Lepidoptera, are very elaborate, consisting of raised lines, ridges, or striæ with transverse ridges between. “The striæ of the transparent scales of Micropteryx are from about 500 to 300 to the millimetre, varying in different species. The 195opaque scales of Morpho, which show metallic reflections, have about 1400 striæ to the millimetre.” (Kellogg.)

The primary use of scales, as observed by Kellogg, is to protect the body, as seen in Synaptera and Lepidoptera. A nearly as important use is the production of colors and patterns of colors and markings, while in certain butterflies certain scales function as the external openings of dermal scent-glands, and they afford in some cases (as first claimed by Kettelhoit in 1860) generic and specific characters. Spuler has shown that the scales are strengthened by internal chitinous pillars. Burgess has observed in the scales of Danais plexippus that the under surface of the scales is usually smooth, or provided with few and poorly developed ridges, and this has been confirmed by Spuler and by Mayer (Fig. 226).

In the irised and metallic scales the ridges, says Spuler, are not divided into teeth, and they converge at the base to the pedicel and also toward the end of the scale (Micropteryx), or end in a single process beyond the middle (the brass-colored scales of Plusia chrysitis).

The arrangement of the scales on the wings is, in the generalized moths, irregular; in the more specialized forms they are arranged in bands forming groups, and in the most specialized Lepidoptera they are more thickly crowded, overlapping each other and inserted in regular rows crossing the wings, these rows either uniting with each other or running parallel. (Spuler.) The scattered irregular arrangement seen in Micropteryx is also characteristic of the Trichoptera and of Amphientomum.

Fig. 223.—Portion of a longitudinal section through one of the young pupal wings of a summer pupa of Vanessa antiopa: s, young scale; leu. cy., leucocyte; mbr. pr., ground membrane; prc, hypodermis-cells.

Fig. 224.—Portion of a longitudinal section through one wall only of the pupal wing of a specimen slightly older than that of Fig. 223; s, older scale.

Development of the scales.—The mode of origin of the scales was first worked out by Semper in 1886, who stated that in the wing of the pupal Sphinx and Saturnia they are seen, in sections, to arise from large roundish cells just under the hypodermis and which have a projection which passes out between the hypodermis (his “epidermis”) cells, expanding into a more or less spherical vesicle, the latter being the first indication of the future scale. He 196observed that the scales are not all formed at once, but arise one after another, so that on one and the same wing the scales are in different stages of development.

Fig. 225.—Portion of a longitudinal section through a pupal wing about eight days before emergence: s, formative scale-cell; upper s, a scale.

More recently Schaeffer has stated that the scales and also the hairs are evaginations of greatly enlarged hypodermis cells, and still more complete evidence has been afforded by A. G. Mayer (1896). In the wings of Lepidoptera, about three weeks before the imago emerges, certain of the hypodermis cells, which occur at regular intervals, begin to increase in size and to project slightly above the level of the hypodermis; these are Semper’s “formative cells,” and are destined to secrete the scales. They increase in length, and appear as in Fig. 223. In the next stage observed, the projections are much longer (Fig. 224). The hypodermis is now thrown up into a regular series of ridges, which run across the wing. Each ridge, says Mayer, corresponds in position with a row of formative cells, and each furrow with the interval between two adjacent rows. The scales always project from the tops of these ridges. The ground or basal membrane has not participated in this folding, and the deep processes of the hypodermis (prc) that once extended to this membrane have largely disappeared. Figure 225 represents a more advanced stage almost eight days before the emergence of the imago.

The scales are originally filled with protoplasm, which gradually withdraws, leaving behind it little chitinous bars or pillars which serve to bind together the upper and lower surfaces of the scales, and finally the scales become “merely little flattened hollow sacs containing only air.” As Mayer shows (Figs. 226, 227), from the study of scales examined four days before emergence of the butterfly (Danais), “the striations upon the upper surface of the scale are due to a series of parallel longitudinal ridges,” while the under side is usually smooth.

The mode of insertion is seen in Fig. 227. The narrow cylindrical pedicel of the scale is merely, according to Semper, inserted into a minute close-fitting socket, which perforates the wing-membrane, 197and not into a tube, as Landois supposed. Spuler describes a sort of double sac structure or follicle (Schuppenbalg) which receives the hollow pedicel of the scale. This was originally (1860) observed by F. J. Carl Mayer, but more fully examined by Spuler (Fig. 228) though not detected by A. G. Mayer.

Fig. 226.—Portion of a cross-section through the pupal wing of Danais plexippus, about six days before emergence: sg, scale; cta.al, wing-membrane; cl.frm, formative cell of the scale; mbr.pr, ground-membrane; fbr.h′drm, hypodermal fibres of pupal wings. A, portion of a longitudinal section through the pupal wing, eight or nine days before emergence; prc, processes of young hypodermis scales.—This and Figs. 223–225 after Mayer.

Spinules, hair-scales, hair-fields, and androconia.—Besides the scales, fine spinules occur on the thickened veins of the wings of the Blattidæ, where they resemble fir-cones; also in the Perlidæ, in the Trichoptera, and in the more generalized Lepidoptera (Micropterygidæ and Hepialidæ), occur, as indicated by Spuler, delicate chitinous hollow spinules scarcely one-tenth as long as, and more numerous than, the scales, which sometimes form what he calls “Haftfelds,” or holding areas. These spinules have also been noticed by Kellogg, and by myself in Micropteryx; Kellogg, and also Spuler, have observed them in certain Trichoptera (Hydropsyche). 198These also occur on the veins, and detached ones near large one-jointed hairs, or hair-scales, said by Kellogg to be striated. Kellogg has detected these scale-hairs, as he calls them, in Panorpa.

Fig. 227.—View looking down upon the upper (i.e. exposed) surface of one of the large scales situated on the veins of Danais plexippus, about four days before emergence: clm, chitinous pillars found in scales. A, a smaller scale, a, a′, sections of the scales. B, leucocyte found in the larger scale.—After Mayer.

Fig. 228.—Scale-follicles: A, of a scale of Galleria mellonella: r, neck-ring. B, the same of Polyommatus phlæas. C, the same of a hair on inner edge of hind wing of Lycæna alexis ♀.—After Spuler.

Fig. 229.—A, portion of wing of a caddis-fly (Mystacides). B, enlarged, showing the androconia and hair-scales. C, a separate androconium.—After Kellogg.

The “hair-scales” of the phylogenetically older Trichoptera correspond to certain scales of Lepidoptera, especially the Psychidæ (Spuler), variously called “plumules” (Deschamps), “battledore scales,” also certain minute cylindrical hairs. To these scent-scales 199is applied the term androconia. They are found, almost without exception, on the upper side of the fore wings, occurring in limited areas, such as the discal spots, or on folds of the wings. Fritz Müller has shown that they function as scent-scales, and are confined to the males. Kellogg has detected androconia-like scales on the wings of a caddis-fly, Mystacides punctata (Fig. 229).

Fig. 280.—Cross-section of androconia surface on wing of Thecla calanus; a, androconia; gl, gland of base; s, ordinary scales; w, wing in section.—After Thomas.

Thomas has proved by sections of the wing of Danais, etc., that the androconia arise from glands situated in a fold of the wing (Fig. 230), and he states that the material elaborated by the local glands, and distributed upon the surface of the wing by the androconia, is that which gives to many of the Lepidoptera their characteristic odor. On comparing these “glands,” it is evident that they are groups of specialized formative cells of Semper (trichogens), which secrete an odorous fluid, issuing perhaps from extremely fine pore-canals at the ends of the androconia. They thus correspond to the glandular hairs, poison-hairs, and spines of caterpillars, the formative cells of which contain either a clear lymph or poison.

LITERATURE

a. Hairs, bristles, cleaning spines, calcaria, combs, etc.

Leydig, Franz. Zum feineren Bau der Arthropoden. (Müller’s Archiv f. Anat. und Phys., 1855, pp. 376–480.)

Fobel, Auguste. Les fourmis de la Suisse. Bâle, 1874.

Saunders, Edward. Remarks on the hairs of some of our British Hymenoptera. (Trans. Ent. Soc. London, 1878, pp. 169–171.)

Perez, J. Notes d’apiculture. (Bull. Soc. d’Apic. de la Gironde, Bordeaux, 1882.)

Osten Sacken, C. R. von. An essay on comparative chætotaxy, or the arrangement of characteristic bristles of Diptera. (Trans. Ent. Soc. London, 1884, pp. 497–517.)

—— Preliminary notice of a subdivision of the suborder Orthorrhapha Brachycera (Diptera) on chætotactic principles. (Berlin Ent. Zeitschr., 1896, pp. 365–373.)

200Janet, Charles. Études sur les fourmis. 8^e Note. Sur l’organe de nettoyage tibio-tarsien de Myrmica rubra. (Ann. Soc. Ent. France, 1895, pp. 691–704, 6 Figs.)

See also J. B. Smith’s Economic Entomology, 1896, hairs of bees. Also the writings of De Geer, Huber, Fenger, Mayr, Forel, Canestrini, and Berlese (1880); Dahl, Cheshire, etc.

b. Glandular and poisonous setæ and spines

Ratzeburg, J. Th. Ch. Ueber entomologische Krankheiten. (Stettin Ent. Zeit., 1846, vii, pp. 35–41.)

Zeller, P. C. Revision der Pterophoriden. (Linnæa Ent., vi, pp. 319–416, 1852, at p. 356 speaks of “Drüsenhärchen.”)

Dimmock, George. On some glands which open externally on insects. (Psyche, iii, pp. 387–401, 1882.)

Goossens, Th. Des chenilles urticants. (Ann. Soc. Ent. France, 1881, pp. 231–236.) Des chenilles vésicants. (Ibid., 1886, pp. 461–464.)

Packard, A. S. Notes on some points in the external structure and phylogeny of insects. (Proc. Boston Soc. Nat. Hist., xxv, 1890, pp. 83–114, 2 Pls.)

—— A study of the transformations and anatomy of Lagoa crispata, a bombycine moth. (Proc. Amer. Phil. Soc., xxxii, pp. 275–292, 7 Pls., 1894.)

Holmgren, Emil. Studier öfner hudens och de körtelartade hudorganens morfologi hos Skandinaviska macrolepidopterlarver. (K. Svenska Vetenskaps-Akad. Handl., xxvii, pp. 1–83, Stockholm, 1895, 9 Pls.)

Also the writings of Leydig, Keller, Bach, Karsten, Scribner, Riley, etc.

(See also Literature of repugnatorial glands.)

c. Androconia

Deschamps, Bernard. Récherches microscopiques sur l’organisation des ailes der Lépidoptères. (Ann. des Sc. nat. [?], iii, pp. 111–157, 1835.)

Waufor, T. W. On certain butterfly scales characteristic of sex. (London, 1867–68.)

McIntire, S. J. Notes on the minute structure of the scales of certain insects. (London, 1871.)

Anthony, J. The markings on the battledore scales of some of the Lepidoptera. (London, 1872.)

Scudder, S. H. Antigeny or sexual dimorphism in butterflies. (Proc. Amer. Acad. Arts and Sc., xii, 1877, pp. 150–158.) Also Butterflies, etc. (New York, 1881, pp. 192–206, figs.)

Müller, Fritz. A prega costal das Hesperideas. (Archivas do Museo nac. do Rio de Janeiro, iii, pp. 41–50, 2 Pls., 1878.)

Thomas, M. B. The androconia of Lepidoptera. (Amer. Nat., xxvii, pp. 1018–1021, 2 Pls., 1893.)

d. Scales

Leydig, Franz. Zum feineren Bau der Arthropoden. (Archiv f. Anat. und Phys., 1855, pp. 376–480, 1 Taf.)

Semper, Carl. Beobachtungen über die Bildung der Flügel, Schuppen, und Haare bei den Lepidopteren. (Zeitschrift f. wissensch. Zoologie, 1857, pp. 326–339, 1 Taf.)

Mayer, F. T. Karl. Ueber den Staub der Schmetterlingsflügel. (Allgem. mediz. Centralzeitung, 1860, pp. 772–774.)

201Landois, H. Beiträge zur Entwicklungsgeschichte der Schmetterlingsflügel in der Raupe und Puppe. (Zeitschr. f. wissensch. Zoologie, xxi, 1871, pp. 305–316, 1 Taf.)

Weismann, August. Ueber Duftschuppen. (Zool. Anzeiger, i, 1878, pp. 98–99.)

Dimmock, George. Scales of Coleoptera. (Psyche, iv, pp. 1–11, 23–27, 43–47, 63–71, 1883.)

Schaeffer, Cäsar. Beiträge zur Histologie der Insekten. (Zool. Jahrbücher, Abth. f. Anat. u. Ontog., iii, pp. 611–652, 2 Pls., 1889.)

Kellogg, Vernon L. The taxonomic value of the scales of the Lepidoptera. (Kansas Univ. Quart., iii, pp. 45–89, figs. 1–17, 9 Taf., 1894.)

Mayer, Alfred G. The development of the wing-scales and their pigment in butterflies and moths. (Bull. Mus. Comp. Zool., xxix., 1896, pp. 209–236, 7 Pls.)

Spuler, Arnold. Beiträge zur Kenntniss des feineren Baues und der Phylogenie der Flügeltedeckung der Schmetterlings. (Zool. Jahrb. Abth. f. Anat. u. Ontog., viii, pp. 520–543, 1 Taf., 1895.)

—— Ueber das Vorhandensein von Schuppenbalg bei den Schmetterlingen. (Biol. Centralblatt, xvi, Sept. 15, 1896, pp. 677–679, 3 figs.)

THE COLORS OF INSECTS

The colors and bright markings of insects, especially those of butterflies, render them the most brilliant and beautiful creatures in existence, rivalling and even excelling the gay hues of our most splendidly colored birds. The subject has been but recently taken up and is in a somewhat crude condition, but the leading features have been roughly sketched out by the work of a few observers from a physical, chemical, and biological point of view.

The colors of insects, as of all other animals, are primarily due to the action of light and air; other factors are, as Hagen observes, heat and cold, moisture and dryness, as recently shown by the experiments on butterflies by Dorfmeister, Weismann, W. H. Edwards, and later observers. They have their seat in the integument. Hagen divides colors into optical and natural.

Optical colors.—“These,” says Hagen, “are produced by the interference of light, and are by no means rare among insects, but they are solely optical phenomena. Colors by the interference of light are produced in two different ways: either by thin superposed lamellæ, or by many very fine lines or small impressions in very close juxtaposition.

“1. There must be present at least two superposed lamellæ to produce colors by interference. The naked wings of Diptera, of dragon-flies, and of certain Neuroptera often show beautiful interference colors. The wings of Chrysopa and Agrion show interference colors only for a certain time, viz., as long as the membranes of the wings are soft and not firmly glued together. Afterwards such wings become simply hyaline.

202“The scales of Entimus and other Curculionidæ are well known for their brilliancy, and it is interesting to remark that when dry scales are examined with the microscope, many are found partly injured, which give in different places different colors, according to the number of layers which remain. The elytra of some Chrysomelina and other beetles with iridescent colors probably belong to the same category.

“2. When there are scales with many fine lines or small impressions close to each other, we have the second mode of producing colors.

“The fine longitudinal and transversal lines of lepidopterous scales seem to serve admirably well to produce the brilliant effect of color-changing butterflies. But there must be something more present, as most of the scales of Lepidoptera are provided with similarly fine lines, and only comparatively few species change colors. I remark purposely that the lines in the color-changing scales are not in nearer juxtaposition.” (Hagen.)

“The colors of butterflies change mostly from purple to blue, sometimes to yellow. The splendid violet color at the end of the wings of Callosune ione is brought out by a combination of the natural with interference colors. Originally the scales are colored lake-red; but a blue interference color is mixed with it; hence the violet hue results. The blue tones, i.e. the splendid varying blue of the Morpho butterflies, Schatz claims, owe their hue less to the interference of light than to a clouded layer of scales situated over the dark ground, through which the light becomes reflected on the same. The scales of the Morphids are in reality brown, as we see by transmitted light; moreover, only the upper side of the scales sends off blue reflections—the under side is simply brown. But the blue scales of Urvilliana are also shining blue beneath; by transmitted light they appear as if clear yellow. The smaragd-green scales of Priamus show by transmitted light a bright red-orange, and the orange-yellow of Crœsus a deep grass-green.” (Schatz in Kolbe.)

“Krukenberg presumes the golden-green color of Carabus auratus to be an interference color. It is not changed by the interference of light, nor was he able to extract from the elytra any green pigment with ether, benzol, carbon of sulphur, chloroform, or alcohol, even after having previously submitted the elytra to the influence of muriatic acid or ammonia. Chlorophyll is not present, whether free or combined with an acid.” (Hagen.)

Leydig has shown that the interference colors of the hairs of certain worms (Aphrodite and Eunice) may be produced by very small impressions in juxtaposition, which bring about the same effect as striæ. Such an arrangement occurs on the feathers of birds, i.e. on the necks of pigeons and elsewhere, and Hagen suggests that this kind of interference colors occurs more frequently among insects than is commonly known. At least the limbs of certain forms appear yellow, but when held in a certain position change to brown or blackish. “I know of no other explanation of this not uncommon fact on the legs of Diptera, of Hymenoptera, and of Phryganidæ.” Interference colors, he adds, may occur in the same place together with natural colors. “The mirror spots of Saturnia pernyi show besides the interference colors a white substance in the cells of the matrix, which Leydig believes to be guanin. But this fact is denied by Krukenberg for the same species and also for Attacus mylitta and Plusia chrysitis.”

203Natural colors.—These are divided by Hagen into dermal (cuticular) and hypodermal. The dermal colors are due to pigment deposited in the form of very small nuclei in the cuticula. Hagen considers them as “produced mostly by oxidation or carbonization, in consequence of a chemical process originating and accompanying the development and the transformations of insects.”

“To a certain extent the dermal colors may have been derived from hypodermal colors, as the cuticula is secreted by the hypodermis, and the colors may have been changed by oxidation and air-tight seclusion. The cuticula is in certain cases entirely colorless,—so in the green caterpillar of Sphinx ocellata; but the intensely red and black spots of the caterpillar of Papilio machaon belong to the cuticula, and only the main yellow color of the body to the hypodermis.” (Leydig, Histiol., p. 114.)

“The dermal colors are red, brown, black, and all intermediate shades, and all metallic colors, blue, green, bronze, copper, silver, and gold. The dermal colors are easily to be recognized as such, because they are persistent, never becoming obliterated or changed after death.” (Hagen.)

Minot and Burgess refer to the cuticular colors of the cotton-worm (Aletia), the dark brown color belonging to the cuticula or crust. “Upon the outside of the crust is a very thin but distinct layer, which in certain parts rises up into a great number of minute, pointed spines that look like so many dots in a surface view. Each spine is pigmented diffusely, and together they produce the brown markings. The spines are clustered in little groups, one group over each underlying hypodermal cell.” (U. S. Ent. Comm., 4th Report, p. 46.) Minot also shows that in caterpillars generally a part of the coloration is caused by pigmentation of the cuticula.

In a dull-colored insect, such as the Mormon cricket (Anabrus), the coloration, as Minot states, depends principally upon the pigment of the hypodermis shining through the cuticula. “Most of the cells contain dull, reddish-brown granules, but scattered in among them are patches of cells bright green in color. I have observed no cells intermediate in color; on the contrary, the passage is abrupt, a brown or red cell lying next a green one. Indeed, I have never seen any microscopic object more bizarre than a piece of the epidermis of Anabrus spread out and viewed from the surface.” (2d Report U. S. Ent. Comm., p. 189.)

The pigment may extend through the entire cuticula, but it is usually confined to the outermost layers, and occurs there in union with a peculiar modelling of the upper surface into microscopic figures which are of interest not only from their delicacy, but because they vary with each species. (See p. 184.)

The hypodermal colors, situated in the hypodermis, are, according to Hagen, the result of a chemical process, generating color out of substances contained in the body. They are easily recognized, since 204they fade, change, and disappear after death. But where these colors are preserved after death and enclosed in air-tight sacs, as in the elytra and scales and hairs of the body, they persist, though, as we well know, they may fade after exposure to light.

The hypodermal colors are mostly brighter and lighter than the dermal ones, being light blue or green in different shades, yellow to orange, and the numerous shades of these colors combined with white; exceptionally they are metallic, as in Cassida, and are then obliterated after death.

“The fact that such metallic colors can be retained in dead specimens by putting a drop of glycerine under the elytra, leads us to conclude that those colors are based upon fat substances. The hypodermal colors are never glossy, as far as I know; the dermal colors frequently.

“As the wings, elytra, and hairs all possess a cuticula, dermal colors are frequently to be found, together with hypodermal ones, chiefly in metallic colors. In the same place both colors may be present, or one of them alone. So we find hypodermal colors in the elytra of Lampyridæ. In the elytra of the Cicindelidæ the main metallic color is dermal, the white lines or spots are hypodermal, by which arrangement the variability in size and shape of those spots is explained.

“There occur in a number of insects external colors, that is, colors upon the cuticula, which I consider to be in fact displaced hypodermal colors: the mealy pale blue or white upon the abdomen of some Odonata, the white on many Hemiptera, the pale gray on the elytra and on the thorax of the Goliath beetle, and the yellowish powder on Lixus. Some of these colors dissolve easily by ether or melt in heat, and some of them are a kind of wax. I believe that those colors are produced in the hypodermis, and are exuded through the pore-canals.” (Hagen.)

The white colors are simply for the most part due to the inclusion of air in scales. The white mother-of-pearl spots of Argynnis are produced by a system of fine transverse pore-canals filled with air; in Hydrometra the white ventral marks have the same origin. (Leydig.)

The further statements and criticisms of Hagen regarding the relation of color to mimicry, sexual selection, and the origin of patterns are of much weight and will be referred to under those heads. Indeed, these subjects cannot well be discussed without reference to the fundamental facts stated in the masterly papers of Leydig and of Hagen, and much of the theorizing of these latter days is ill-founded, because the colors of insects and animals are attributed to natural selection, when they seem really the result of the action of the primary factors of organic evolution, such as changes of light, heat, cold, and chemical processes dependent on the former.

As to the chemical nature of color, Hagen, after quoting the results of Krukenberg and others, thinks that the colors of insects are chemically produced by a combination of fats or fat-acids with other 205acids or alkalis under the influence of air, light, and heat. He concludes:—

1. That some colors of insects can be changed or obliterated by acids.

2. That two natural colors, madder-lake and indigo, can be produced artificially by the influence of acid on fat-bodies.

3. As protein bodies in insects are changed into fat-bodies, and may be changed by acids contained in insects into fat-acids, the formation of colors in the same manner seems probable.

4. That colors can be changed by different temperatures.

5. That the pattern is originated probably by a combination of oxygen with the integument.

6. That mimicry of the hypodermal colors may be effected by a kind of photographic process.

7. Finally, color and pattern are produced by physiological processes in the interior of the bodies of insects.

Krukenberg concludes that change of color (in perfectly developed insects) is a consequence of the change of food, and can be explained by the alteration of the pigment through heat and light. His experiments were made in order to ascertain the cause of the turning of green grasshoppers in autumn into yellow and pink. He tried to answer two questions: First, does the pigment of grasshoppers originate directly out of the food, and does it consist of pure chlorophyll or of a substance containing chlorophyll, or is it to be accepted as a peculiar product of the organism? Second, is the color the consequence of only one pigment, or of several? Special analysis proves that the green color has no connection with chlorophyll. He concludes: “It is evident that the green color of the grasshopper is the consequence of several different pigments which can be separated by a chemical process.” Krukenberg believes that light has a marked influence on the color of insects and that light turns to red or pink the insects which were green during the summer. It would seem, however, more probable that cold was the agent, the change being due to the colder autumn weather.

Here we might refer to the results of the studies of Buckton and Sorby, on the changes in color of Aphides:—

“1. The purple coloring matter appears to be a quasi-living principle, and not a product of a subsequent chemical oxidizing process. Mounted in balsam or other preserving fluids, the darker species stain the fluid a fine violet.

“2. As autumn approaches and cold weather reduces the activity of the Aphides, the lively greens and yellows commonly become converted into ferruginous red, and even dark brown, which last hue in reality partakes more or less of intense violet or purple. These changes have some analogy with the brilliant hues assumed by maple and other leaves during the process of slow decay.

“3. Aqueous solutions of crushed dark brown and yellow-green varieties of Aphides originate different colors with acids and alkalies.

“4. In the generality of cases coloring-matters, such as indigo, Indian yellow, madder-lake, and the like, do not separately exist in the substance of vegetables, but the pigments are disengaged through fermentation or oxygenation. Again, alizarin itself is reddish yellow, but alkaline solutions strike it a rich 206violet just as we find them to act towards the substance which Mr. Sorby calls aphidilutein.

“5. Mr. Sorby’s four stages of the changes effected by the oxidation of aphideine produce four different substances.”

Chemical and physical nature of the pigment.—Researches in this difficult field of inquiry have been made by Landois (1864), Sorby (1871), Meldola (1871), by Krukenberg (1884), and more recently by Coste, Urech, Hopkins, and Mayer, and the subject is of fundamental importance in dealing with mimicry and protective coloration, the primary causes of which appear to be due to the action of physical and chemical agents.

Over twenty years ago Meldola observed that the yellow pigment of the sulphur-yellow butterfly (Gonopteryx rhamni) was soluble in water, and showed that its aqueous solution had an acid reaction.

Besides the yellow uranidin found by Krukenberg in different beetles and lepidopterous pupæ, still other coloring-matters, which are very constant in different species are readily recognized by the spectroscope. “Thus there appear in the brownish yellow lymph of Attacus pernyi, Callosamia promethea and Telea polyphemus, after saponification of the precipitated soap readily effected by ether, or incompletely or not removed by benzine, a chlorophane-like lipochrome; and in the yellowish green lymph of Saturnia pyri and of Platysamia cecropia besides this pigment still another whose spectrum shows a broad band on D, but which disappears with the addition of acetic acid or ammonia, as also after a long heating of the lymph up to 66° C.”

Coste, and more especially Urech, have shown that many of the pigments may be dissolved out of the scales by means of chemical reagents, giving colored solutions, and leaving the scales white or colorless. They have also shown that some of these pigments may be changed in color by the action of reagents, and then restored to their original color by other reagents. They have proved that reds, yellows, browns, and blacks are always due to pigments, and in a few cases greens, blues, violets, purples, and whites, and not, as is usually the case, to structural conditions, such as striæ on the scales (Mayer). They confined themselves solely to the chemical side of the problem, not considering the structure of the scales themselves.

Urech has also discovered a beautiful smaragd-green coloring-matter in the wings (not in the scales) of the pupa of Pieris brassicæ. It is not chlorophyll, and Urech suggests that it may be either the germinal substance of the pigments of the scales or its bearer. It is not the pigment of the blood.

Urech has also demonstrated that in many Lepidoptera the color of the urine which is voided upon emergence from the chrysalis is similar to the principal color of the scales.

207Hopkins has worked on the pigments within the scales of butterflies. The yellow pigment in Gonopteryx rhamni is a derivation of uric acid, and he calls it lepidotic acid. Its aqueous solution is strongly acid to litmus, and must be bad-tasting to birds.

Hopkins has dissolved the red pigment from the border of the hind wing of Delias eucharis, an Indian butterfly, in pure water, finding as the result a yellow solution; but if the solution be evaporated to dryness, the solid residue of pigment is red once more. He has obtained from this pigment of eucharis a silver compound which contains a percentage of metals exactly equal to that from the pigment of G. rhamni. (Nature, April 2, 1892.)

“The scales of the wings of the white butterflies (Pieridæ) are also shown by Hopkins to contain uric acid, this substance practically acting as a white pigment in these insects. A yellow pigment, widely distributed in the same family, is shown to be a derivative of uric acid, and its artificial production as a by-product of the hydrolysis of uric acid is demonstrated. That this yellow pigment is an ordinary excretory product of the butterfly is indicated by the fact that an identical substance is voided from the rectum on emergence from the pupa. These excretory pigments, which have well-marked reactions, are apparently confined to the Pieridæ, and are not found in other Rhopalocera. This fact shows that when a Pierid mimics an insect belonging to another group, the pigments of the mimicked and mimicking insects, respectively, are chemically quite distinct. Other pigments existing, not in the scales, but between the wing-membranes, are shown to be of use for ornament.” (Proc. Royal Soc., London, 1894.)

Griffiths (1892) claims that the green pigment found in several species of Papilio, Hesperia, and Limenitis, also in Noctuidæ, Geometridæ, and Sphingidæ likewise consists of a derivative of uric acid, which he calls lepidopteric acid. By prolonged boiling in HCl it is converted into uric acid.

Spuler, however, finds that green does not depend on pigmentation, but is an optical color. As remarked by Spuler, either the chitin of the scales itself is colored reddish (yellow grayish), or the pigment is secreted in the nuclei.

A. G. Mayer believes that the pigments of the scales are derived from the hæmolymph or blood of the pupa, for the following reasons: (1) He is unable to find anything but blood within the scales during the time when the pigment is formed. (2) In Lepidoptera generally the first color to appear upon the pupal wings is a dull ochre-yellow, or drab, and this is also the color assumed by the blood when it is removed from the pupa and exposed to the air. (3) He has succeeded by artificial means in manufacturing several pigments from the blood which are similar in color to various markings upon the wing of the imago; chemical reagents have the same effect upon these manufactured pigments that they do upon the similarly colored pigments of the wings. “It should be here noted,” he says, “that in 1866 Landois pointed out the fact that the color of the dried blood of many caterpillars is similar to the ground color of the wings of the mature insect.”

Ontogenetic and phylogenetic development of colors.—The colors of the wings of Lepidoptera, as is well known, are acquired at the end 208of the pupal state. The order of development of the colors in the pupal wings has been observed by Schaeffer, Van Bemmelen, Urech, Haase, Dixey, Spuler, and A. G. Mayer. The immature wings are at first transparent and full of protoplasm. The transparent condition of the wings corresponds to the period before the scales are formed, and when they are full of protoplasm; they then become whitish as the scales develop; the latter are at first filled with protoplasm, and afterwards turn whitish, being little hollow sacks filled with air. After the protoplasm has completely withdrawn from the scales, the blood of the pupa enters them, and then the coloring-matter forms. (Mayer.) He adds that “about twenty-four hours after the appearance of the dull yellow suffusion the mature colors begin to show themselves. They arise, faint at first, in places near the centre of the wings, and are distinguished by the fact that they first appear upon areas between the nervures, never upon the nervures themselves. Indeed, the last place to acquire the mature coloration are the outer and costal edges of the wings, and the nervures.”

The faint color of the scales gradually increases in intensity. “For example, if a scale be destined to become black, it first becomes pale grayish brown, and this color gradually deepens into black.”

Urech states that in Vanessa io first a white, and in V. urticæ a pale reddish hue, are spread over the entire wings, and then successively arise other colors in the following order: yellow, yellow to brown, red, brown and black.

Spuler, however, claims that the differentiation of colors and markings do not follow one another, but arise simultaneously, and that his view is confirmed by Fischer. This may be the case with the highly specialized and diversely marked butterflies, but certainly taking the Lepidoptera as a whole the yellows and drabs must have been the primitive hues, the other colors being gradually added in the later more specialized forms.

It is noticeable that the most generalized moths, such as the species of Micropteryx, Tinea, Psychidæ, Hepialidæ (in general), etc., are dull brown or yellow-drab without bars, stripes, or spots of bright hues. These shades prevail in others of the more primitive Lepidoptera, such as many bombycine moths, and they even appear to a slight extent in certain caddis-flies. The authors mentioned, especially Mayer, whom we quote, claim that “dull ochre-yellows and drabs are, phylogenetically speaking, the oldest pigmental colors in the Lepidoptera; for these are the colors that are assumed by the hæmolymph upon mere exposure to the air. The more brilliant pigmental colors, 209such as bright yellow, reds, greens, etc., are derived by more complex chemical processes. We find that dull ochre-yellow and drabs are at the present day the prevalent colors among the less differentiated nocturnal moths. The diurnal forms of Lepidoptera have almost a monopoly of the brilliant colorations, but even in these diurnal forms one finds that dull yellow or drab colors are still quite common upon those parts of their wings that are hidden from view.”

The more primitive moths being more or less uniformly yellowish or drab, the next step was the formation of bars, stripes, finally spots, and eyed spots, these markings in the later forms appearing simultaneously in one and the same species of certain highly specialized moths and butterflies. All that has been said will prepare the reader for the consideration of the subject of insect coloration. The origin of such markings has been discussed by Weismann, Eimer, Haase, Dixey, Fischer, and others.

LITERATURE

Heer, O. Einfluss des Alpenklimas auf die Farbe der Insecten. (Froebel u. Heer, Mitth. aus dem Gebiete der theoret. Erdkunde, 1836, i, pp. 161–170.)

Goureau. Mémoire sur l’irisation des ailes des insectes. (Ann. Soc. Ent. France, 2 sér., i, 1848, pp. 201–215.)

Laboulbène, A., et M. Follin. Note sur la matière pulvérulente qui recouvre la surface du corps des Lixus et de quelques autres insectes. (Ann. Soc. Ent. de France, 1848, vi, pp. 301–305, Fig.)

Coquerel, Ch. Note sur la prétendue poussière cryptogamique qui recouvre le corps de certains insectes. (Ann. Soc. Ent. France, 1850, viii, pp. 13–15.)

Brauer, F. Beobachtungen in Bezug auf den Farbenwechsel bei Chrysopa vulgaris. (Verhandl. k. k. zool.-botan. Gesellsch. Wien., 1852, pp. 12–14.)

Prittwitz, O. F. W. v. Bemerkungen über die geographische Farbenverteilung unter den Lepidopteren. (Stett. Ent. Zeit., 1855, xvi, pp. 175–185.)

Latham, A. G. The causes of the metallic lustre of the scales on the wings of certain moths. (Proc. Lit. and Phil. Soc. Manchester, iii, 1864, pp. 198–199. Quart. Journ. Micr. Sc., new ser., iv, 1864, pp. 48–49.)

Sorby, H. C. On the coloring matter of some Aphides. (Quart. Journ. Micr. Sc., new ser., xi, 1871, pp. 352–361.)

Leydig, Franz. Bemerkungen über Farben der Hautdecke und Nerven der Drüsen bei Insekten. (Archiv f. mikr. Anatomie, xii, 1876, pp. 536–550, 1 Taf.)

Weismann, A. Studien zur Descendenz-Theorie, ii, 1876.

Hemmerling, Hermann. Ueber die Hautfarbe der Insecten. (Bonn, 1878, p. 27.)

Buckton, C. B. Monograph of the British Aphides. (London, 1879, ii, p. 167.)

Cameron, P. Notes on the coloration and development of insects. (Trans. Ent. Soc. London, 1880, pp. 69–79.)

Hagen, Hermann A. On the color and pattern of insects. (Proc. Amer. Acad. Arts and Sc., 1882, pp. 234–267.)

210Poulton, Edward Bagnall. The essential nature of the colouring of phytophagous larvæ (and their pupæ), etc. (Proc. Roy. Soc. London, xxxviii, pp. 269–315, 1884–1885.)

—— An inquiry into the cause and extent of a special colour-relation between certain exposed lepidopterous pupæ and the surfaces which immediately surround them. (Phil. Trans. Roy. Soc. London, clxxviii, pp. 311–441, 1 Pl., 1887.)

Krukenberg, C. Fr. W. Grundzüge einer vergleichenden Physiologie der Farbstoffe und der Farben. (Heidelberg, 1884, pp. 102.)

McMunn, C. A. Krukenberg’s chromatological speculation. (Nature, xxxi, p. 217, 1885.)

Müller, Fritz, and Dr. H. A. Hagen. The color and pattern of insects. (Kosmos, xiii, 1886, pp. 466–469.)

Slater, J. W. On the presence of tannin in insects and its influence on their colors. (Trans. Ent. Soc. London, 1887, iii, Proceed., pp. 32–34.)

Bemmelen, J. F. van. Ueber die Entwicklung der Farben und Adern auf den Schmetterlingsflügeln. (Tijdschrift der nederland. Dierkundige Vereeniging, ser. 2, pp. 235–247, 1889.)

Hopkins, F. G. Uric acid derivatives functioning as pigments in butterflies. (Proc. Chem. Soc. London, 1889, p. 117; also Nature, xl, p. 335.)

—— Pigment in yellow butterflies. (Nature, xlv, p. 197, 1891.)

—— The pigments of the Pieridæ. (Proc. Roy. Soc. London, lvii, No. 340, pp. 5, 6, 1894. Phil. Trans. Roy. Soc. London, clxxxvi, pp. 661–682, 1896.)

Coste, F. H. P. Contributions to the chemistry of insect colors. (The Entomologist, xxiii, 1890; xxiv, 1891, pp. 9–15, etc. Nature, xlv., pp. 513–517, 541–542, 605.)

Urech, F. Beobachtungen über die verschiedenen Schuppenfarben und die zeitliche Succession ihres Auftretens. (Zool. Anzeiger, xiv, pp. 466–473, 1891; Ibid., August 1, 1892.)

—— Beiträge zur Kenntniss der Farbe von Insektenschuppen. (Zeits. f. Wissens. Zool., lvii, pp. 306–384, 1893.)

Griffiths, A. B. Recherches sur les couleurs de quelques insectes. (C. R. Acad. Sc. Paris, cxv, pp. 958, 959.)

Mayer, Alfred Goldsborough. On the color and color-patterns of moths and butterflies. (Proc. Bost. Soc. Nat. Hist., xxvii., March, 1897, pp. 243–330, 10 Pls. See also p. 201 under Mayer.)

Also the writings of Bates, Beddard, Belt, Butler, Darwin, Dimmock, Dixey, Eimer, Haase, Higgins, Müller, Poulton, Seitz, Wallace, Weismann.

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2. INTERNAL ANATOMY

THE MUSCULAR SYSTEM

In its general arrangement the muscular system of insects corresponds to the segmented structure of the body. Of the muscles belonging to a single segment, some extend from the front edge of one segment to that of the next behind it, and others to the hinder edge; there are also sets of dorsal and ventral muscles passing in an oblique or vertical course (Figs. 16–18). As Lang observes, “the greater part of the muscles of the body can be traced back to a paired system of dorsal and ventral intersegmental longitudinal muscles.” The muscular system is simplest in larval insects, such as caterpillars, where the musculature is serially repeated in each segment.

In the larva of Cossus Lyonet found on one side of the body 217 dorsal, 154 lateral, 369 ventral, and in the thoracic legs 63, or 803 muscles in all. “Adding to this number the 12 small muscles of the second segment, and 8 others of the third, which he did not describe, there would be for all the muscles on one side of the caterpillar 823. This would make for the entire body 1646, without counting a small single muscle which occurs in the subdivision of the last segment,” and also those of the internal organs as well as those of the head, so that the total number probably amounts to about 2000, not 3000, as usually stated in the books. Lubbock admits that Lyonet was right in his mode of estimating the number. In the larva of Pygærci bucephala he found that “the large muscles scarcely vary at all,” though certain smaller ones are very variable. Lubbock observed that certain of the longitudinal muscles in the caterpillar of Diloba split up into numerous, not less than ten, separate fascicles. “This separation of the fibres composing a muscle into separate fascicles is carried on to a much greater extent in the larvæ of Coleoptera. Of course in the imago the number of thoracic muscles is greatly increased, or at least in Dyticus and the wood-feeding Lamellicorns, which alone I have examined. In these two groups each of the larger muscles is represented by at least twenty separate fascicles, which makes it far more difficult to distinguish the arrangement of the muscles.”

The muscles are whitish or colorless and transparent, those in the thorax being yellowish or pale brown; and of a soft, almost gelatinous consistence. In form they are simply flat and thin, straight, band-like, or in rare cases pyramidal, barrel or feather shaped. They act variously as rotators, elevators, depressors, retractors, protractors, flexors, and extensors.

212

Fig. 231.—Diagram of the muscles and nerves of the ventral surface of the segments in the larva of Sphinx ligustri: A, A, recti muscles; 1, 2, ventral recti muscles (1, recti majores; 2, recti minores); 3, ridge giving origin to recti muscles of one segment, and insertion to the same of the adjoining segment; 4, ridge for attachment of muscle; 5, retractor ventriculi, connecting the mid-intestine with the outer integument of the body. B, 6, first oblique,—7, second oblique,—9, 10, third oblique, muscles; 11, fourth oblique,—12. third rectus,—13, fifth oblique,—14, triangularis, muscle; 15, transversus medius; 16, transverse ridge; 17, transversi abdominales; 18, abdominales anteriores; 19, 20, abdominales laterales, some (20) longer than others; 21, obliquus posterior; 22, postero-laterales obliqui; 23, transversus lateralis; 24, second transversus lateralis; 25, retractor spiraculi, or constrictor of the spiracles, attached by a long tendon (26); 27, retractor valvulæ. Nerves: a, ganglion,—c, transverse nerves, of which p is the first, q the second, r the third, and s the fourth branch; t, the main trunk, which crosses the great longitudinal trachea, receives a filament from the transverse nerve (n), and divides into two branches (t);—some of these branches form a small plexus (u); the nerve t divides in two divisions (p and v). The second division ends in w and x; the branch q divides into y and z. For other explanations, see Newport, art. Insecta.—After Newport.

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Fig. 232.—Musculature of the European cockchafer, Melolontha vulgaris: a, a, levatores capitis; b, depressores capitis; c, rotatores capitis; d, depressors externi; e, retractor or flexor of the jugular plate; f, oblique extensor of the jugular plate; g, the other retractor of the jugular plate; h, retractor prothoracis superior; i, inferior retractor, the proper depressor of the prothorax; k, elevator prothoracis; l, one of the rotatores prothoracis; m, n, o, flexors of the coxa; x, great depressor muscle of the wing; y, y, elevators and protractors attached to the metaphragma and base of the postfurca; z, second flexor of hind leg; a, a, extensors of hind leg; c, c, dorsal recti of abdomen. Q, ejaculatory duct; R, penis; S, its prepuce. M, rectum.—After Straus-Durckheim, from Newport.

Our knowledge of the muscular system of insects is still very imperfect. To work it out thoroughly one should begin first with that of Scolopendrella, then some generalized synapterous form, as Japyx or Lepisma, then passing to that of a caterpillar, and ending with some of the more highly specialized forms, such as a beetle, etc. Thus far our knowledge is confined to that of the caterpillars (Lyonet, Newport, and Lubbock) and the beetle (Straus-Durckheim) and ants (Forel, Lubbock, and Janet).

Musculature of a caterpillar.—Newport’s account of that of the larva of Sphinx ligustri is the most useful (Fig. 231). The muscles here present, he says, great uniformity of size and distribution in every segment, the motions of each of these divisions of the body being almost precisely similar, especially in the 4th to 9th trunk segments. In these segments the first layer seen on removing the fat and viscera are the flat straight recti muscles. They are the most powerful of all the trunk muscles, and are those which are most concerned in shortening the body, in effecting the duplicature of the external teguments during the changes of the insect, and which during the larval state mainly assist in locomotion. There are four sets, two dorsal and two ventral (Fig. 231, A, A). Without entering into farther details, the reader is referred to the works of Newport and to Fig. 231.

Musculature of a beetle.—The best general account of the musculature of a perfect insect is that of Straus-Durckheim in his famous work on the Melolontha. 214We will copy the summary of Newport, who adopted the nomenclature applied to these parts by Burmeister:—

“The muscles that connect the head with the thorax are contained within the prothorax (Fig. 232, 2), and are of three kinds, extensors, flexors, and retractors. The extensors, levatores capitis (a, a), consist of two pairs, one of which arises from the middle line of the pronotum, and diverging laterally from its fellow of the opposite side, passes directly forwards, and is inserted by a narrow tendon into the anterior superior margin of the occipital foramen. The other arises further back from the prophragma. It is a long, narrow muscle that passes directly forwards through the prothorax, and is inserted by a tendon near the superior median line of the foramen; so that, while this muscle and its fellow of the opposite side elevate the head almost in a straight line, the one first described, when acting alone or singly, draws the head a little on one side; but when the whole of these muscles act in unison, they simply elevate the head upon the prothorax. The depressors or flexors, depressores capitis (b), are exceedingly short muscles, which arise from the jugular plate, or, when that part does not exist, from the border of the prosternum, and are attached to the inferior margin of the occipital foramen. They simply flex the head on the prothorax. The lateral flexors, depressores externi (d), are two little muscles that arise from the same point as the preceding, and are attached to the lateral inferior margin of the occipital foramen. The rotatory muscles, rotatores capitis (c), are two flat muscles like the elevators, which arise, one at the side of the antefurca and the other from the posterior jugular plate, and passing upwards and outwards are attached to the lateral margin of the occipital foramen. The retractor or flexor of the jugular plate is a small muscle (e) that arises from the margin of the antefurca, and passing directly forwards is inserted by a small tendon into the middle of the jugular piece. The oblique extensor of the jugular plate is a long, slender muscle (f) that arises from the external margin of the pronotum, and passing obliquely downwards and forwards traverses the prothorax and is inserted by a narrow tendon to the jugular plate immediately before the retractor. The other retractor (g) arises from the anterior superior boundary of the pronotum, and passing downwards is inserted into the jugular plate between the larger levator and flexor capitis.

“The muscles proper to the prothorax consist of four pairs, by which it is united to the succeeding segments. The first of these, the superior retractor, retractor prothoracis superior (h), arises by a broad, fleshy head from the anterior external margin of the pronotum, and passing directly backwards is inserted by a tendon into the prophragma, a little on one side of the median line. The next muscle of importance, the inferior retractor (i), arises from the anterior border of the medifurca, and is united to the posterior of the antefurca, thus forming with that muscle part of the great recti of the larva. This muscle must be considered as the proper depressor of the prothorax. The elevator prothoracis (k) is narrow, pyramidal, and arises fleshy from the lateral surface of the prophragma. It passes downwards and is attached by a narrow tendon to the superior portion of the antefurca. The rotatores prothoracis are the largest of all the muscles of this segment. They arise, one on each side (l), by a narrow head from the posterior part of the pronotum, and passing beneath the prophragma are considerably enlarged and attached to the tegument between the two segments, and also to the anterior portion of the mesothorax. The remaining muscle proper to the prothorax is the closer of the spiracle, an exceedingly small muscle not shown in the drawing.

“The other muscles of this segment are those of the legs, which are of considerable size. There are three distinct flexors of the coxa (m, n, o). The first of 215these arises from the superior lateral border of the pronotum, the second from the superior posterior border, the third from the sides of the prothorax, and the fourth a little nearer posteriorly, and the whole of them are attached by narrow tendons to the sides of the coxa. But there is only one extensor muscle to this part. In like manner, the extensor of the trochanter is formed of three portions (Fig. 233, a, b, c); but there is only one flexor (d), and one abductor (e). In the femur, there is one extensor (f),—a long penniform muscle that occupies the superior part of the thigh, and is attached by a tendon to the anterior-posterior margin of the joint formed by the end of the tibia. There is also but one flexor (g) in the femur, which, like the preceding muscle, is penniform, and occupies the inferior portion of the femur, and its tendon is attached to the inferior border of the tibia. In the tibia itself there is also one flexor and one extensor. The flexor (i) occupies the superior portion of the limb, and ends in a long tendon (l) that passes directly through the joints of the tarsus, on their inferior surface, and is attached to the inferior margin of the claw (g). The extensor (h) occupies the inferior portion of the tibia, and is shorter than the preceding muscle, like which it ends in a long tendon that is attached to the upper margin of the claw. Besides these muscles, which are common to the joints of the tarsus, there are two others belonging to the claw, situated in the last joint. The first of these, the extensor (m), is short, and occupies the superior portion of the last phalanx of the tarsus, and the other, the flexor (n), is a much longer penniform muscle, which occupies nearly the whole of the upper and under surface of the posterior part of the phalanx, and is attached, like the long flexor of the tarsus, to the inferior part of the claw.”

Fig. 233.—Muscles of the fore leg of Melolontha vulgaris: a, b, c, three divisions of the extensor of the trochanter; d, flexor,—e, abductor, of the trochanter; f, extensor of the femur; g, flexor of the femur; h, extensor of the tibia; i, flexor of the tibia; l, tendon attached to the lower edge of the claw (g); m, extensor,—n, flexor, of the claw.—After Straus-Durckheim, from Newport.

These are the muscles of the prothorax, and its organs of locomotion. The reader is referred for a further account of the muscles of the hinder thoracic and of the abdominal segments to Straus-Durckheim’s original work.

Minute structure of the muscles.—The muscular fibres of insects are striated (Figs. 235–238), even those of the alimentary canal; the only notable exception being the alary muscles of the pericardial septum, while Lowne states that certain of the thoracic muscles of the blow-fly are not striated (Miall and Denny).

216

Fig. 234.-Section through the prothorax of Diapheromera femoratum: prov, proventriculus; tr, trachea; n. c, nervous cord; s. gl, salivary gland; hyp, hypodermis; ur. t, urinary tube; ht, heart; m, m″, m‴, muscles for lowering and raising the tergum; m′, another muscle, its use unknown.

Fig. 235.—Striated muscular fibre of Hydrophilus: A and B, two fibrillæ in a state of extension; a, thick disk; b, thin disk; c, intermediate space. C, D, portion of the same fibrillæ seen by moving the objective farther away and using a small diaphragm; n, thick; c, thin disk. × 2000 diam.—After Ranvier, from Perrier. E after Gehuchten, from Lang.

In describing the minute structure of the muscles of ants, wasps, and bees, C. Janet states that each consists of a group of fibres diverging from a tendon, which is an integumentary invagination (Fig. 236). Each fibre may be regarded as a multinucleate cell; 217the sarcolemma represents the cell-membrane. It forms a resistant and extremely elastic tube. The longitudinal (Fig. 236, E) and radiating filaments or reticulum (spongioplasm of Gehuchten) lie in a nutritive filling substance (the hyaloplasm of Gehuchten). The radiating filaments are formed of an exceedingly elastic substance, and serve to sustain the longitudinal filaments, to transmit the nervous stimulus to them, and to bring them back into position after contraction. Janet’s account agrees on the whole with that of Gehuchten.

Fig. 236.—Preparations from the adductor muscle of the mandible of Vespa crabro, worker, fixed by heat and alcohol several hours after leaving its cell. A to E × 425; F × 212: A, terminal cupule of the tendon of a fibre. B, C, union of the fibres with their tendon. D, branch of the tendon of a muscle sending out tendons of some of the fibres; this branch is accompanied with numerous nervous ramifications (N). E, fragment of a nerve which furnishes the ramifications of Fig. D. F, fragment of the tendon of the adductor muscle of the mandible; at the left are seen the terminal cupules of the fibres (td, c); on the right, on the body of the tendons, some sessile cupules, each of which forms the attachment of a fibre; td, b, tendons of the fibres.—After Janet.

The muscles of flight are said to be penetrated by fine tracheal branches, probably to supply a greater amount of oxygen, as the most energetic movements of the insect are made in moving the wings during flight; while the other muscles of the body are only surrounded by the air-tubes. (Sharp.)

Without entering into tedious details, the reader is referred to figures or references to the more important systems of muscles, such as those of the legs and other appendages, of the wings, of respiration, etc., to the sections treating of those organs or functions; also to Figs. 16, 17, 18, 22, 48, 74, 81, 83, 84, 115, 116, 172, 173, 174, etc.

Muscular power of insects.—The most detailed and careful experiments are those of Plateau. His experiments prove that even the 218weakest insects pull at least five times their own weight; many of them, however, get the better of a burden twelve to twenty fold as heavy as themselves, while a strong man or a draught horse, for example, is not even able to pull a burden which is equal to the weight of his body. Plateau came to the following results as to the relation of the weight of the body to the load drawn (1 and 2 are to be compared with each other, 1 being the larger, and 2 the smaller insect; it will be seen that the smaller insect is the stronger).

Fig. 237.—Vespa crabro, worker, fixed by heat and alcohol some hours after leaving its cell. A × 425; B to D × 850 times: A, muscular fibre of the motor muscles of the mandibles treated, for ten minutes, by 1 per cent potassium to bring out the reticulum; the nodes of union of the rayed filaments with the longitudinal filaments are indicated by distinct granulations (l.d), and these longitudinal filaments present accessory thickenings (d.a); T, trachea; N, junction of a nervous filament with the muscular fibres. B, fibre of the same muscle, not treated with potassium, stained by hæmatoxylin; C, transverse section of a disk at the level of a layer of rayed filaments; Sarc, sarcolemma. D, transverse section of a disk at the level of the rods; nuc, nucleus.—After Janet.

1. Carabus auratus 17.4.

2. Nebria brevicollis 25.3.

1. Cetonia aurata 15.

2. Trichius fasciatus 41.3.

1. Melolontha vulgaris 14.3.

2. Anomala frischii 24.3.

1. Oryctes nasicornis 4.7.

2. Geotrupes stercorarius 9.8.

3. Onthophagus nuchicornis 14.4.

1. Necrophorus vespillo 15.1.

2. Silpha livida 24.4.

1. Ocypus morio 17.

2. Quedeus fulgidus 29.6.

1. Donacia nymphææ, 42.7.

2. Crioceris merdigera 39.2.

1. Bombus terrestris 16.1.

2. Bombus rupestris 14.5.

3. Apis mellifica 20.2.

As regards the pushing power, the relation of the load to the size of the body in different large beetles, gave the following figures:—

Oryctes nasicornis 3.2.

Geotrupes stercorarius 28.4.

Onthophagus nuchicornis 92.9.

219The leaping force of locusts was found by Straus-Dürckheim to be in Œdipoda grossa as 1.6, in Œ. parallela as 3.3 of their weight.

Fig. 238.—Vespa crabro, fixed and stained as in the subjects of the other figures. I, N, P × 1700; H, J, M × 850; the others × 425 times: A-C, motor muscles of the antennal scape. D-P, motor muscles of the 3d coxa. A, B, the two ends, in very different states of contraction, of the same fibre; on one side the transverse striæ are near together, on the other very far apart. C, a crushed and split fibre showing a fibrous appearance, owing to the rupture of the radiated filaments, and the separation of the longitudinal filaments. D, muscular disk seen in section, with two rows of nuclei. E, a muscular fibre with three rows of nuclei. F, a nucleus, accompanied with coagulated protoplasm, oozing from a previous break of the muscular fibre. G, nerve-terminations very near each other on the same muscular fibre. H, longitudinal filaments, evenly covered with the coagulated substance, and forming, throughout the mass of the fibre, continuous filaments. I, filaments widely separated. J, longitudinal filaments showing the beginning of one of the transverse breaks which isolate some of the disks. K, oblique view of a disk obtained by such a break, and of a fibre in circular section, with an axial row of nuclei; this piece comprises three stages of radiated filaments. L, muscular fibre with a row of nuclei; at the lower part, the nuclei have issued from a longitudinal fissure in the fibre, and have remained attached in a chain. M, edge of fibre in which there is quite a large, clear space between the sarcolemma and the rods. N, passage of the trachea, with the spiral thread, into three capillaries with a smooth cuticula. O, elliptical disk from a fibre, with two rows of nuclei, and showing a layer of radiated filaments. P, fragment (highly magnified) of the edge of a disk seen in section.—After Janet.

A humble bee (Bombus terrestris) can carry while flying a load 0.63 of its own weight, and a honey bee 0.78; here, as usual, the smaller insect is the stronger.^[39]

220

LITERATURE ON THE MUSCLES

a. General

Lyonet, P. Traité anatomique de la chenille. La Haye, 1762.

Cornalia, E. Monographia del Bombyce del gelso. (Mem. R. Instituto Lombardo Sc. Lett. ed Arte, 1856.)

Basch, S. Skelett und Muskeln des Kopfes von Termes. (Zeitschr. f. wissens. Zool., xv, 1865, pp. 55–75, 1 Taf.)

Lubbock, John. Arrangement of the cutaneous muscles of the larva of Pygæra bucephala. London, 1858. 2 Pls.

—— On some points in the anatomy of ants. (Month. Micr. Journ., xviii, pp. 121–142, 1877, 4 Pls.)

—— On the anatomy of ants. (Trans. Linn. Soc., Ser. 2; Zool., ii, 1879, pp. 141–154, 2 Pls.)

Poletajeff, N. Du développement des muscles d’ailes chez les Odonates. (Horæ Soc. Ent. Ross., xvi, 1879, pp. 10–37, 5 Pls.)

—— Die Flugmuskeln der Lepidopteren und Libelluliden. (Zool. Anzeiger, 1880, pp. 212, 213.)

—— Ueber die Flugmuskeln der Rhopaloceren. (Arbeiten d. Russ. Ent. Ges., 1881, xiii, p. 9, 1 Taf., in Russian.)

Lendenfeld, R. von. Der Flug der Libellen. (Sitzb. k. Akad. Wissens., 1 Abth. Wien, 1881, lxxxiii, pp. 289–376, 7 Taf.)

Luks, Constantine. Ueber die Brustmuskulature der Insekten. (Jena. Zeitschr. f. Naturwissen., xvi, N. Folge IX, 1883, pp. 520–552, 2 Taf.)

Carlet, G. Sur les muscles de l’abdomen de l’abeille. (Comptes rend., 1884, xcviii, pp. 758, 759.)

Janet, Charles. Sur les muscles des fourmis, des guêpes et des abeilles. (Comptes rend., cxxi, p. 610, 1 Fig., 1895.)

Also the writings of Straus-Durckheim, Newport, Graber, Burgess, Leydig, Dahl, Ockler, Dogiel, Dimmock, Kraepelin, Becher, Langer, Kolbe.

b. Histology

Aubert, H. Ueber die eigenthümliche Struktur der Thoraxmuskeln der Insekten. (Zeitschr. f. wissens. Zool., iv, 1853, pp. 388–399, 1 Taf.)

Verson, E. Zur Insertionsweise der Muskeln. (Sitzsb. Akad. d. wiss. math. naturw. Cl. Wien., lvii, 1 Abth., pp. 63–66, 1868.)

Künckel d’Herculais. Sur le développement des fibres musculaires striées chez les insectes. (Compt. rend. de l’Acad. Sc. Paris, lxxv, 1872.)

Grunmach, Emil. Ueber die Structur der quergestreiften Muskelfaser bei den Insekten. Berlin, 1872. pp. 47.

Fredericq, L. Note sur la contraction des muscles striés de l’Hydrophile. (Bull. Acad. Roy. Belgique, xli, p. 583, 2 Pls.)

Gehuchten, A. van. Étude sur la structure intime de la cellule musculaire striée. (La Cellule, ii, pp. 289, 293–453, 1886, 6 Pls.)

Janet, Charles. Études sur les fourmis, les guêpes et les abeilles. 12^e note. (Structure des membranes articulaires des tendons et des muscles, Limoges, 1895, pp. 25, 11 Figs.)

Also the writings of Burmeister, Chabrier, Leydig, Meckel, Lebert, Wagner, Wagener, Amici, Krause, Heppner, Retzius, Rollet, G. Elias Müller, F. Merkel, Hensen, Kölliker, Dogiel, Dönitz, Hagen, Vosseler, Bütschli u. Schewiakoff, Lowne, Ciaccio, Biedermann, Cohnbeim, Brücke, Haycraft, Melland, Bowman.

221

c. Muscular power of insects

Plateau, Félix. Sur la force musculaire des insectes. (Bull. Acad. Roy. Belgique, 2 Sér. xx, 1865, pp. 732–757; xxii, 1866, pp. 283–308.)

—— Recherches sur la force absolue des muscles des invertébrés. 1884.

Radan, R. La force musculaire des insectes. (Revue de deux mondes, 2 Sér., lxiv, 1866, pp. 770–777.)

Bibiakoff, Paul von. Zur Muskelkraft der Insekten. (Natur, xvii, 1868, p. 399.)

Delbœuf. Nains et géants, Étude comparative de la force des petits et des grands animaux. Bruxelles. (Also in Kosmos, xiii, 1883, pp. 58–62.)

Camerano. Mem. Acc. Torino (2), xliii, 1893, p. 229.

Also Newport, Art. Insecta, p. 76. Kirby and Spence, Burmeister, Graber, Kolbe, pp. 375, 376.

222

THE NERVOUS SYSTEM

a. The nervous system as a whole

Fig. 239.—Central nervous system of Machilis maritima: au, eye; lo, optic tract; g, brain; an, antennal nerve; oe, œsophagus passing between the œsophageal commissures; usg, infraœsophageal ganglion; I-III, thoracic ganglia; 1–8, abdominal ganglia, the last (Sabc) consisting of three fused ganglia; s, sympathetic nervous system of the ventral cord.—After Oudemans, from Lang.

The nervous system of insects consists of a double series or chain of ganglia connected by nervous cords or commissures. The first of these is the brain or supraœsophageal ganglion; it is situated in the upper part of the head, above the gullet or œsophagus, while the rest of the system, called the ventral cord, lies on the floor of the body, under the digestive canal.

A ganglion or nerve-centre consists of a mass of ganglion-cells, from each of which a process or fibre passes off, uniting with others to form a nerve; by means of these nerves the ganglia are connected with other ganglia, and with the sensory cells and muscle-fibres. The ganglia may be simple, and arranged in pairs, corresponding to each segment of the body, or they may be compound, the result of the fusion of several pairs of ganglia, which in the early stages of the embryo are separate. Thus the brain of insects is a compound ganglion, or ganglionic mass.

The nerves are of two kinds: 1. Sensory, which transmit sensations from the peripheral sense-cells to the ganglion, or brain; 2. Motor, which send stimuli from the brain or any other ganglion to the muscles.

Of ganglion cells, some are tactile, and others give rise to nerves of special sense, being distributed to the eyes, or to the organs of hearing, smell, taste, or touch.

223

Fig. 240.—Nervous system of Melanoplus spretus: sp, supraœsophageal ganglion, sending off the large optic nerve (op) to the eyes, and an ocellar nerve to each ocellus (the dotted line oc stops short of the left ocellus); if, infraœsophageal ganglion; 1, 2, 3, thoracic ganglia; 1–5, five abdominal ganglia (the fifth the largest, and sending branches to the ovipositor, etc.) The sympathetic nerve and ganglia are represented by the two main nerves which arise from the medio-cephalic (as) resting on and above the œsophagus, and two ganglia (ps) on the under side of the crop. From each of these ganglia, two nerves are sent under the crop, and a larger nerve on each side to as far as the stomachal cæca, ending the figure at the dotted line 2, near the second thoracic ganglion. u, a round, shining body, connected by a nerve with the medio-cephalic ganglion, its nature unknown.

Fig. 241.—Section through the head of Machilis, showing the brain (br), and subœsophageal ganglion (soe. g); cl, clypeus; lbr, labrum; oc, ocellus.

While the supraœsophageal ganglion, or “brain,” of the insect is much more complex than any other ganglion, consisting more exclusively both of sensory as well as motor ganglia and their nerves, it should be borne in mind that the subœsophageal ganglion also receives nerves of special sense, situated on the palpi and on the tongue, as in the bee and other insects; hence this ganglion is probably complex, consisting of sensory and motor cells. The third thoracic ganglion is also, without doubt, a complex one, as in the locusts the auditory nerves pass into it from the ears, which are situated at the base of the abdomen, while in the green grasshoppers, such as the katydids and their allies, whose ears are situated in their fore legs, the first thoracic ganglion is a complex one. In the cockroach and in Leptis (Chrysopila), a common fly, the caudal appendages bear what are probably olfactory organs, and as these parts are undoubtedly supplied from the last abdominal ganglion, this is probably composed of sensory and motor ganglia; so that we have in the ganglionated cord of insects a series of brains, as it were, running from head to tail, and thus in a still stronger sense 224than in vertebrates the entire nervous system, and not the brain alone, is the organ of the mind of insects.

The simplest, most primitive form of the nervous system of insects is seen in that of the Thysanura. That of Campodea has not yet been fully examined, but in that of the more complicated genus, Machilis (Fig. 239), we see that there is a pair of ganglia to nearly each segment, while the brain (Fig. 241) is composed of three lobes, viz. the optic, the cerebral (Fig. 239, g), behind which is the antennal lobe, from which the antennal nerve takes its origin. Behind the opening for the throat (oe) is situated the first ganglion of the ventral cord, the subœsophageal ganglion, which gives rise to the nerves supplying the jaws and other mouth-parts.

Fig. 242, A-D.—The nervous systems of 4 genera of Diptera, to demonstrate their various degrees of fusion of ganglia: A, non-concentrated more primitive nervous system of Chironomus plumosus, with 3 thoracic and 6 abdominal ganglionic masses. B, nervous system of Empis stercorea, with 2 thoracic and 5 abdominal ganglionic masses. C, nervous system of Tabanus bovinus, with 1 thoracic ganglionic mass, and the abdominal ganglia closely approximated. D, highly modified nervous system of Sarcophaga carnaria, in which all the ganglia of the ventral cord behind the subœsophageal ganglion are fused into a single ganglionic mass.—After Brandt, from Lang.

In the Collembola, which are retrograde Thysanura, there are from one (Smynthurus), to three or four ventral ganglia.

In the winged insects, where the ganglia are more or less fused, the fusion taking place in the head and at the end of the abdomen; there are in the more simple and generalized forms, such as Ephemera, the grasshopper, locusts (Fig. 240), etc., thirteen ganglia besides the two pairs of compound ganglia in the head, three pairs of thoracic 225ganglia, and usually from five to eight pairs of ganglia in the abdomen.

Fig. 243.—Nervous system of the May beetle, Lachnosterna fusca: w¹, nerve to 1st,—w², nerve to 2d, pair of wings; ig, infraœsophageal ganglion.

Fig. 244.—The same of the stag-beetle, Lucanus dama, where there are 3 thoracic, and 3 separate abdominal ganglia.

In certain winged insects the process of fusion or degeneration is carried to such an extreme that there are either no abdominal ganglia (Fig. 242, D), or their vestiges are situated in the thorax and partially fused with the thoracic ones, as in the May beetle, in which the prothoracic pair of ganglia is separate, while the two other thoracic ganglia are fused with the abdominal, the latter being situated in the thorax; this fusion is carried to a further extent than in any other Coleoptera yet examined. In many Diptera and Hemiptera the abdominal ganglia are either absent or the vestiges are fused with the thoracic ganglia.

Rhizotrogus, which is allied to our May beetle, as also Hydrometra and the Stylopidæ are said to lack the subœsophageal ganglion (Brandt).

In numerous Coleoptera (Acilius, Gyrinus, Necrophorus, Melolontha, Bostrichus, Rhynchænus); in many Diptera (Culex, Tipula, 226Asilus, Xylophaga, and Phora); and in the higher Hymenoptera (Crabronidæ, Vespidæ, and Apidæ), as well as in many Lepidoptera (Vanessa, Argynnis, and Pontia), two of the thoracic ganglia are fused together, while all three are partially fused into a single mass in many brachycerous Diptera (Conops, Syrphus, Pangonia, and the Muscidæ); in certain Hemiptera (Pentatoma, Nepa, and Acanthia); also in a beetle (Serica brunnea). Sometimes the subœsophageal ganglion is fused with the first thoracic, as in Acanthia, Nepa, and Notonecta. The greatest amount of variation is seen in the number of abdominal ganglia, all being fused into a single one or from one to eight. The fusion is usually greatest where the abdomen is shortened, due to the partial atrophy and modification of the terminal segments which bear the ovipositor, where present, and the genital armature.

There is only one pair of abdominal ganglia in Gyrinus and in certain flies (Conops, Trypeta, Ortalis, and Phora); two in Rhynchænus, a weevil, and in the flies, Syrphus and Volucella; three in Crabro and Eucera; four in Sargus, Stratiomys and in butterflies, five in the beetle, Silpha, and in the fly, Sciara, and the moth, Hepialus.

The nervous system in the larvæ of the metabolous orders is not concentrated, though in that of the neuropterous Myrmeleo it has undergone fusion from adaptation to the short compressed form of this insect.

b. The brain

The brain of insects appears to be nearly, if not quite, as complex as that of the lower vertebrates. As in the latter, the pair of supraœsophageal ganglia, or brain, is the principal seat of the senses, the chief organ of the insect’s mind.

It is composed of a larger number of pairs of primitive ganglia than any of the succeeding nerve-centres, and is, structurally, entirely different from and far more complicated than the other ganglia of the nervous system. It possesses a central body in each hemisphere, a “mushroom body,” optic lobes and optic ganglia and olfactory lobe, with their connecting and commissural nerve-fibres, and a number of other parts not found in the other ganglia.

In the succeeding ganglia the lobes are in general motor; the fibres composing the œsophageal commissures, and which arise from the œsophageal commissural lobes, extend not only to the subœsophageal ganglion, but pass along through the succeeding ganglia to the last pair of abdominal nerve-centres.^[40] Since, then, there is a 227direct continuity in the fibres forming the two main longitudinal commissures of the nervous cord, and which originate in the brain, it seems to follow that the movements of the body are in large part directed or coördinated by the brain.^[41] Still, however, a second brain, so to speak, is found in the third thoracic ganglion of the locust, which receives the auditory nerves from the ears situated in the base of the abdomen; or in the first thoracic ganglion of the green grasshoppers (katydids, etc.), whose ears are situated in their fore legs; while even the last pair of abdominal ganglia in the cockroach and mole cricket, is, so to speak, a secondary brain, since it distributes sensory nerves to the caudal stylets, which are provided with organs probably olfactory in nature.

It is impossible to understand the morphology of the brain unless we examine the mode of origin of the nervous system in the early life of the embryo. The head of an embryo insect consists of six segments, i.e. the ocular, antennal, premandibular, mandibular, and the 1st and 2d maxillary segments, so named from the appendages they bear. Of these the first three in the larva and adult are preoral, and the last three are postoral. The antennal segment was probably either postoral in the progenitors of insects, or the antennæ were inserted on the side of the mouth, the latter finally moving back.^[42]

The nervous system in the early embryonic condition, as shown by Wheeler (Fig. 245), at first consists of nineteen pairs of primitive ganglia, called neuromeres. Those of the head, which later in embryonic life fuse together to form the brain, are the first three, corresponding to the protocerebrum, deutocerebrum, and tritocerebrum 228of Viallanes. The first pair of primitive ganglia, and which is situated in front of the mouth, is divided into three lobes.

Fig. 245, A-D.—Diagrams of four consecutive stages in the development of the brain and nerve-chain of the embryo of Xiphidium: I, cephalic,—II, thoracic,—III, abdominal, region; st, stomodæum or primitive mouth; an, anus; e, optic plate; pc(og), 1st protocerebral lobe, or optic ganglion; pc², pc³, 2d and 3d protocerebral lobes; dc, deutocerebrum; tc, tritocerebrum; 1–16, the 16 postoral ganglia; po. c, postoral commissure; fp, furcal pit; ac, anterior,—pc, posterior, ganglionic commissure; ag, anterior,—pg, posterior,—cg, central,—lg, lateral gangliomeres.—After Wheeler.

The first or outermost lobe, according to Wheeler, forms the optic ganglion of the larva and imago, while the second and third lobes. (pc², pc³) ultimately form the bulk of the brain proper, or the protocerebral lobes. The second (primitively postoral) brain-segment or pair of ganglia gives origin to the antennæ, while the third brain, or premandibular (intercalary) segment, gives origin to a temporary embryonic pair of appendages found in Anurida and Campodea (the premandibular ganglia), and also to the nerves supplying the labrum. These three pairs of ganglia later on in embryonic life become preoral, the mouth moving backwards. The three pairs of primitive ganglia, behind, i.e. the mandibular and 1st and 2d maxillary ganglia, become fused together to form the subœsophageal ganglion, and which in larval and adult life is postoral.

229If the tongue (ligula, or hypopharynx) represents a distinct pair of appendages, then there are seven segments in the head.

Fig. 246.—Section through head of a carabid, Anopthalmus telkampfii: br, brain; fg, frontal ganglion; soe, subœsophageal ganglion; co, commissure; n. l, nerve sending branches to the lingua (l); mn, maxillary nerve; mx, 1st maxilla; mm, maxillary muscle; mx′ 2d maxilla; mt, muscle of mentum; le, elevator muscle of the œsophagus; l′ of the clypeus, and a third beyond raising the labrum (lbr); eph, epipharynx; g. g′, salivary glands above; g², lingual gland below the œsophagus (œ); m, mouth; pv, proventriculus; md, mandible.

The brain, then, supplies nerves to the compound and simple eyes, and to the antennæ, and gives origin to the sympathetic nerves; it is thus the seat of the senses, also of the insect’s mind, and coördinates the general movements of the body.

Fig. 247.—Median longitudinal section through the head of Blatta orientalis. The nervous system of the head is drawn entire. hyp, hypopharynx; os. oral cavity; lbr, upper lip; gf, frontal ganglion; g, brain; na, root of the antennal nerve; no, root of the optic nerve; ga, anterior,—gp, posterior ganglion of the paired visceral nervous system; œ, œsophagus; c, œsophageal commissure; usg, infraœsophageal ganglia; cc, longitudinal commissure between this and the first thoracic ganglion; sg, common duct of the salivary glands; lb, labium (2d maxillæ); nr, recurrent nerve; d, nerve uniting the frontal ganglion with the œsophageal commissure; e, nerve from this commissure to the labrum; f, nerve from the infraœsophageal ganglion to the mandible, —g, to the 1st maxillæ, —h, to the lower lip (2d maxillæ).—After Hofer, from Lang.

231

Fig. 248.—1, front view of the brain of Melanoplus femur-rubrum: opt. gang, optic ganglion; oc, ocelli and nerves leading to them from the two hemispheres, each ocellar nerve arising from the region containing the calices; m. oc, median ocellar nerve; opt. l, optic lobe sending off the optic nerve to the optic ganglion; ant. l, antennal or olfactory lobe; ant. n, antennal nerve; f. g, frontal ganglion of sympathetic nerve; lbr. n, nerve to labrum; x, cross-nerve or commissure between the two hemispheres; œ. c, œsophageal commissure to subœsophageal ganglion. 2, side view of the brain and subœsophageal ganglion (lettering of brain as in 1): s. g, stomatogastric or sympathetic nerve; a. s. g, anterior, and p. s. g, posterior, sympathetic ganglia; g², subœsophageal ganglion; md, nerve to mandible; mx, maxillary nerve; ln, labial nerve; nl, unknown nerve,—perhaps salivary. 3, interior view of the right half of the head, showing the brain in its natural position: an, antenna; cl, clypeus; lbr, labrum; m, mouth-cavity; md, mandible; t, tongue; œ, œsophagus; c, crop; en, right half of the endocranium or X-shaped bone, through the anterior angle of which the œsophagus passes, while the great mandibular muscles play in the lateral angles. The moon-shaped edge is that made by the knife passing through the centre of the X. 4, view of brain from above (letters as before). 5, subœsophageal ganglion from above: t. c, commissure to the succeeding thoracic ganglion (other letters as before). Fig. 3 is enlarged 8 times; all the rest 25 times.—Drawn from original dissections, by Mr. Edward Burgess, for the Second Report of the U. S. Entomological Commission.

The pair of subœsophageal ganglia distributes nerves to the mandibles, to the 1st and 2d maxillæ, and to the salivary glands (Fig. 248).

Its general shape and relations to the walls and to the outer organs of the head is seen in Figs. 247, 248. In all the winged insects (Pterygota) its plane is situated more or less at right angles to the horizontal plane of the ventral cord. On the dorsal and anterior sides are situated the ocular lobes, and below these the antennal lobes.

Viallanes first, independently of embryonic data, divided the brain of adult insects into three regions or segments; i.e. the “protocerebron,” “deutocerebron” and “tritocerebron,” which he afterwards found to correspond with the three primitive elements (neuromeres) of the brain and with the segments of the head of the embryo.

The brain of the locusts (Melanoplus and Œdipoda) being best known will serve as the basis of the following description, taken mainly from Viallanes, with minor changes in the name of the three segments, and other modifications.

I. The optic or procerebral segment is composed of a median portion, i.e. two fused procerebral lobes (median protocerebrum), and of two lateral masses, the optic ganglia (protocerebrum), and comprises the following regions fused together and forming the median procerebral mass (Viallanes):—

1. Procerebral lobes.

2. Optic ganglia.

3. Layer of postretinal fibres.

4. Ganglionic plate. (Periopticon of Hickson.)

5. External chiasma.

6. External medullary mass. (Epiopticon of Hickson.)

7. Internal chiasma.

2328. Internal medullary mass. (Opticon of Hickson.)

9. Optic ganglia and nerves.

10. Pedunculated or stalked body. (Mushroom body of Dujardin.)

11. Bridge of the procerebral lobes.

12. Central body.

Fig. 249.—Diagram of an insect’s brain: cc, central body; cg, ganglionic cells; che, external, chi, internal chiasma; cœ, œsophageal commissure; cp, mushroom body; ctc, tritocerebral commissure; fpr, postretinal fibres; goc, ocellar ganglion; goc¹, œsophageal ganglion, the dotted ring the œsophagus; gv¹, gc², gv³, 1st, 2d, 3d, unpaired visceral ganglion; gvl, lateral visceral ganglion; ld, dorsal lobe of the deutocerebrum; lg, ganglionic plate; lo, olfactory lobe; lpc, protocerebral lobe; me, external, mi, internal medullary mass; na, olfactory or antennal nerve; nl, nerve to labrum; no, ocular nerve; nt, tegumentary nerve; œ, œsophagus; plp, bridge of the protocerebral lobes; rvd, visceral root arising from the deutocerebrum; rvt, visceral root arising from the tritocerebrum; tr, tritocerebrum; to, optic nerve or tract.—After Viallanes.

Optic ganglia.—Each of the two optic ganglia is formed of a series of three ganglionic masses situated between the compound eyes and the median procerebral mass, i.e. the ganglionic plate (Fig. 249, lg), the external medullary mass (me), and the internal medullary mass (mi).

The postretinal fibres (fpr) arising from the facets or single eyes of the compound eye (ommatidia) pass into the ganglionic plate (lg), which is united within by the chiasmatic fibres (che, external chiasma) of the external medullary mass (me). The last is attached to the internal medullary mass (mi) by fibres (chi), some of which are chiasmatic, and others direct. Finally, the internal medullary mass connects with the median part of the protocerebrum by direct fibres forming the optic nerve or tract (to).

Procerebral lobes.—The median procerebral lobes are fused together on the median line, forming a single central mass. From each side 233or lobe arises the mushroom or stalked body. In the middle of the mass is the central body, and directly in front is the procerebral bridge (plp). The latter is a band uniting the two halves of the brain.

The procerebral lobes also give origin to the nerves to the ocelli (no).

Fig. 250.—Transverse section through the brain of the locust (Œdipoda and Caloptenus): c′, lower part of the wall of c, calyx;—st, stalk of the same; bpcl, bridge of the protocerebral lobes; mo, nerve of median ocellus; ch, transverse fascia of the optico-olfactory chiasma; fcb, fibrous region of the central body; lcb, tubercle of the central body; fch, descending fascia of the optico-olfactory chiasma; choo, superior fascia of the optico-olfactory chiasma; pt, protocerebral lobes; ld, dorsal lobe of the deutocerebrum; lt, tritocerebral lobe; gcld, gc, ganglion cells.—After Viallanes.

The mushroom or stalked bodies.—These remarkable organs were first discovered by Dujardin, who compared them to mushrooms, and observed that they were more highly developed in ants, wasps, and bees than in the lower insects, and thus inferred that the higher intelligence of these insects was in direct relation to the development of these bodies. We will call them the mushroom bodies.

These two bodies consist of a rounded lobular mass (the trabecula) of the procerebral lobe, from which arises a double stalk (Fig. 253), the larger called the cauliculus, the smaller the peduncle (or pedicel); these support the cap or calyx. The calices of the bee were compared by Dujardin to a pair of disks on each side of the brain as seen from above, “each disk being folded together and bent downwards before and behind, its border being thickened, and the inner 234portion radiated.” In the locust there are but two divisions of the calyx; in the cockroach, ants, wasps, and bees, four.

The shape and relation of the mushroom bodies are represented in Figs. 252 and 253. The bodies are connected by commissural fibres, and are connected with the optic ganglion of the same side, and with the central body; while they are connected with the antennal lobes by the optico-olfactory chiasma.

Fig. 251.—Sagittal section through the brain of the locust: l. oc. n, lateral ocellus nerve; a. t, anterior tubercle of the mushroom body; i. t, internal tubercle of the mushroom body; c. l, cerebral lobes; l. l, lateral lobe of the middle protocerebrum; com, commissural cord; c. mol, central mass of the olfactory lobe; ac. an. l, fibres uniting the median lobe of the middle protocerebrum with dorsal lobes of the deutocerebrum; gc. trit. l, ganglionated cortex of the tritocerebral lobe; c. an. l, cortex of antennal (olfactory) lobe; lab. fr, labrofrontal nerve; oe. com, œsophageal commissure; tr. com, transverse commissure of œsophageal ring; other letters as in Fig. 250.—After Viallanes.

The stalked bodies are enveloped by the cortical layers of ganglion-cells, those filling the hollow of the calyx having little or no protoplasm around the nucleus.

Structure of the mushroom bodies.—By staining the brain of the honey bee with bichromate of silver, Kenyon has worked out the structure of the mushroom bodies, with their cells. The cup-shaped bodies or calyces are composed of fibrillar substance (punktsubstanz). Each of these cups, he says, is “filled to overflowing with cells having large nuclei and very little cytoplasm.” From the under surface of each of these cups there descends into the general fibrillar substance of the brain “a column of fibrillar substance, which unites with its fellow of the same side to send a large branch obliquely downward to the median line of the brain, and an equally large or larger branch straight forwards to the anterior cerebral surface.”