Chapter VI

CLASSIFICATION—THE NINE ORDERS OF INSECTS—THEIR CHARACTERS—PACKARD'S ARRANGEMENT—BRAUER'S CLASSIFICATION—CLASSIFICATIONS BASED ON METAMORPHOSIS—SUPER-ORDERS—THE SUBDIVISIONS OF ORDERS.

Classification.

We have already alluded to the fact that Insects are the most numerous in species and individuals of all land animals: it is estimated that about 250,000 species have been already described and have had scientific names given to them, and it is considered that this is probably only about one-tenth of those that really exist. The classification in a comprehensible manner of such an enormous number of forms is, it will be readily understood, a matter of great difficulty. Several methods or schemes have since the time of Linnaeus been devised for the purpose, but we shall not trouble the reader to consider them, because most of them have fallen into disuse and have only a historical interest. Even at present there exists, however, considerable diversity of opinion on the question of classification, due in part to the fact that some naturalists take the structure of the perfect or adult Insect as the basis of their arrangement, while others prefer to treat the steps or processes by which the structure is attained, as being of primary importance. To consider the relative values of these two methods would be beyond our scope, but as in practice a knowledge of the structures themselves must precede an inquiry as to the phases of development by which the structures are reached; and as this latter kind of knowledge has been obtained in the case of a comparatively small portion of the known forms,—the embryology and metamorphosis having been investigated in but few Insects,—it is clear that a classification on the basis of structure is the only one that can be at present of practical value. We shall therefore for the purposes of this work make use of an old and simple system, taking as of primary importance the nature of the organs of flight, and of the appendages for the introduction of food to the body by the perfect Insect. We do not attempt to disguise the fact that this method is open to most serious objections, but we believe that it is nevertheless at present the most simple and useful one, and is likely to remain such, at any rate as long as knowledge of development is in process of attainment.

Orders.

The great groups of Insects are called Orders, and of these we recognise nine, viz. (1) Aptera, (2) Orthoptera, (3) Neuroptera, (4) Hymenoptera, (5) Coleoptera, (6) Lepidoptera, (7) Diptera, (8) Thysanoptera, (9) Hemiptera. These names are framed to represent the nature of the wings; and there is some advantage in having the Orders named in a uniform and descriptive manner. The system we adopt differs but little from that proposed by Linnaeus.^[107] The great Swedish naturalist did not, however, recognise the Orders Orthoptera and Thysanoptera; and his order Aptera was very different from ours.

These Orders may be briefly defined as follows,—the reader being asked to recall the fact that by a mandibulate mouth we understand one in which the mandibles, or the maxillæ, or both, are fitted for biting, crushing, or grasping food; while the term suctorial implies that some of the mouth parts are of a tubular form or are protrusible as a proboscis, which assists, or protects, a more minute and delicate sucking apparatus:—

1. Aptera (ἀ without, πτερόν a wing). Wingless^[108] Insects; mouth mandibulate or very imperfectly suctorial. Metamorphosis very little.

2. Orthoptera (ὀρθός straight, πτερόν a wing). Four wings are present, the front pair being coriaceous (leather-like), usually smaller than the other pair, which are of more delicate texture, and contract in repose after the manner of a fan. Mouth mandibulate. Metamorphosis slight.

3. Neuroptera (νεῦρον nerve, πτερόν a wing). Four wings of membranous consistency, frequently with much network; the front pair not much, if at all, harder than the other pair, the latter with but little or no fanlike action in closing. Mouth mandibulate. Metamorphosis variable, but rarely slight.

4. Hymenoptera (ὑμήν membrane, πτερόν a wing). Four wings of membranous consistency; the front pair larger than the hind, which are always small and do not fold up in repose. Mouth mandibulate, sometimes provided also with a tubular proboscis. Metamorphosis very great.

5. Coleoptera (κολεός sheath, πτερόν a wing). Four wings; the upper pair shell-like in consistency, and forming cases which meet together over the back in an accurate line of union, so as to entirely lose a winglike appearance, and to conceal the delicate membranous hind pair. Mouth mandibulate. Metamorphosis great.

6. Lepidoptera (λεπίς scale, πτερόν a wing). Four large wings covered with scales. Mouth suctorial. Metamorphosis great.

7. Diptera (δίς double, πτερόν a wing). Two membranous wings. Mouth suctorial, but varying greatly. Metamorphosis very great.

8. Thysanoptera (θύσανος fringe, πτερόν a wing). Four very narrow fringed wings. Mouth imperfectly suctorial. Metamorphosis slight.

9. Hemiptera (ἡμι half, πτερόν a wing). Four wings; the front pair either leather-like with more membranous apex, or entirely parchment-like or membranous. Mouth perfectly suctorial. Metamorphosis usually slight.

We must again ask the reader to bear in mind that numerous exceptions exist to these characters in most of the great Orders; for instance, wingless forms are not by any means rare in several of the Orders.

Before remarking further on this system we will briefly sketch two other arrangements of the Orders of Insects, for which we are indebted to Packard and Brauer.

Packard's Classification.

Packard has devoted much attention to the subject, and has published two or three successive schemes, of which the following is the most recent:^[109] the definitions are those of the author himself, but the information in brackets is given to institute a concordance with the system we adopt:—

1. Thysanura. Wingless; often with a spring (equivalent to our Aptera).

2. Dermaptera. Front wings minute, elytra-like (= Forficulidae, a part of our Orthoptera).

3. Orthoptera. Wings net-veined; fore wings narrow, hind wings folded (= our Orthoptera after subtraction of Dermaptera).

4. Platyptera. Four net-veined wings; mouth parts adapted for biting (= Termitidae and Mallophaga, parts of our Neuroptera).

5. Odonata. Wings net-veined, equal (= Odonata, a division of our Neuroptera).

6. Plectoptera. Wings net-veined, unequal (= Ephemeridae, a part of our Neuroptera).

7. Thysanoptera. Mouth beaklike but with palpi (= our Thysanoptera).

8. Hemiptera. Mouth parts forming a beak for sucking. No palpi (= our Hemiptera).

The above eight Orders form the group Ametabola, while the following eight constitute the Metabola:—

9. Neuroptera. Wings net-veined; metamorphosis complete (= a small part of our Neuroptera).

10. Mecaptera. Wings long and narrow (for a small part of our Neuroptera; the Panorpatae of Brauer).

11. Trichoptera. Wings not net-veined (= our division of Neuroptera with the same name).

12. Coleoptera. Fore wings sheathing the hinder ones (= our Coleoptera).

13. Siphonaptera. Wingless, parasitic. Flea (= a division of Diptera).

14. Diptera. One pair of wings (= our Diptera after subtraction of Siphonaptera).

15. Lepidoptera. Four wings (and body) scaled (= our Lepidoptera).

16. Hymenoptera. Four clear wings; hinder pair small; a tongue (= our Hymenoptera).

Although this system of the Orders of Insects has some valuable features it is open to very serious objections, to which we can only briefly allude. The Order Hemiptera with its extensive divisions, Heteroptera, Homoptera, Coccidae, and Anoplura exhibiting great differences in structure and considerable divergence in metamorphosis, is treated as only equivalent to the little group Panorpatae (scorpion-flies); these latter being considered a distinct order, although they are not very different in structure or metamorphosis from the Orders he calls Neuroptera and Trichoptera. The arrangement appears to be specially designed with the view of making the Orders adopted in it fall into the two groups Ametabola and Metabola. The propriety of such a course is more than doubtful since very few of the Ametabola are really without metamorphosis, in the wide sense of that term, while the Metabola include Insects with various kinds of metamorphosis. Indeed if we substitute for the term Ametabola the more correct expression, "Insects with little metamorphosis," and for Metabola the definition, "Insects with more metamorphosis but of various kinds," we then recognise that the arrangement is, like all others, a quite artificial one, while it is of little value, owing to the development of so few Insects being hitherto fully ascertained.

Brauer's Classification.

Professor Brauer has recently proposed^[110] to adopt 17 Orders or chief groups of Insects, arranging them as follows:—

The chief characters on which Brauer bases his system are: (1) The existence or absence of wings. (2) The condition of the mouth, and whether it undergoes radical changes in the ontogeny, arriving thus at the categories Menognatha, Metagnatha, and Menorhyncha, as we have mentioned on p. 161. (3) The metamorphosis; the grouping adopted being Ametabola, Hemimetabola, Metabola. (4) The number of the Malpighian tubules; Oligonephria, Polynephria. (5) The nature of the wings, the relative proportions of the thoracic segments, and some other characters.

Brauer's treatise is accompanied by a valuable and in many respects very sagacious consideration of the generalised characters of the Insecta; as a classification based partly on generalisations and partly on structures, it is, so far as the present condition of our knowledge goes, a good one. But it is of little value as a practical guide, and as a basis for theoretical speculation it cannot be treated as of importance, because the generalisations it makes use of are premature, owing to the small proportion of the forms that have been examined. And even now the groups adopted are known to be subject to many exceptions.

Thus it begins by a division of Insecta into winged and wingless; but the winged division is made to comprehend an enormous number of wingless Insects, whole subdivisions of Orders such as the Mallophaga being placed in the winged series, although all are without wings. This first division is indeed entirely theoretical; and if a classification on generalisations were adopted, it would be more natural to begin with the old division into Homomorpha and Heteromorpha, and treat the Order Aptera as the first division of the Homomorpha, while the Heteromorpha would commence with the Ephemeridae and Odonata, in which, though the individual in the early part of the ontogeny is very different from the perfect Insect, there is no marked division of the later larval and the pupal stages. Brauer's system is also defective inasmuch as it takes no account of the embryological or oogenetic processes, though these are of equal importance with the later phases of the Ontogeny. Even as regards the division into Orders, it is far from being free from reproach; for instance, the separation of the Dermaptera from the Orthoptera, while Rhynchota remains intact, although including a more extensive series of heterogeneous forms; the division of the Neuroptera into widely separated groups, each of which is treated as equivalent to the great Orders, such as Coleoptera (in which Strepsiptera are included), Hymenoptera, and Diptera, is not reasonable. The association of Mallophaga and Termitidae, while Dermaptera are separated from Orthoptera, is also undeniably arbitrary, and other similar disparities are to be seen on scrutinising the details of the system.

On comparing the three arrangements we have outlined, it will be seen that the chief discrepancies they present come under two heads: (1) The treatment of the Neuroptera, opinions differing as to whether these Insects shall be grouped as a single Order, or shall be divided into numerous Orders; and as to what, if this latter course be adopted, the divisions shall be. (2) The treatment of the parasitic groups Mallophaga, Aphaniptera, etc. It must be admitted that whichever of the three systems we have sketched be adopted, the result is, as regards both these points, open to criticism. The Order Neuroptera, if we take it in the broad sense, differs from the other Orders in the greater variety of metamorphosis exhibited by its members; while if, on the contrary, it be dismembered, we get a number of groups of very unequal extent and not distinguished from one another by the same decisive and important characters as are the other Orders of which they are considered equivalent. The discrepancy exists in nature, and can scarcely be evaded by any system. A similar observation may be made as to the parasitic groups, viz. Mallophaga, Anoplura, Aphaniptera, and Strepsiptera. If these be treated as separate Orders the result is not satisfactory; while, if they be associated with the larger groups to which they are respectively nearest allied, it is almost equally unsatisfactory.

We may mention that Packard and Brauer have in their treatises discussed the question of super-orders, and have gone so far as to propose names for them. These two authorities do not however agree in their conclusions; and as the names proposed are of little practical value, and are but rarely met with, we need not explain them or discuss the comparative merits of the two systems.

The divisions of inferior value to the Order are, after repeated scrutiny by many naturalists, becoming of a more satisfactory character, and notwithstanding various anomalies, may be, many of them, considered fairly natural.^[111] Unfortunately entomologists have not been able to agree on a system of terminology, so that for these subdivisions terms such as sub-order, series, legion, section, tribe, etc., are used by different authorities in ways so various as to cause much confusion. In the following pages the terms sub-order and series will be used in a somewhat vague manner, the term sub-order being preferred where the group appears to be an important one and of a fairly natural character, while the word series will be adopted when the groups are connected in a conventional manner. The designation "family" will be used for groups of subordinate importance; and as regards this term we may remark that systematic entomologists are making genuine efforts to define the "families" in an accurate and comprehensible manner. The endeavour to make these systematic families dependent throughout the Class Insecta on characters of similar morphological value has, however, scarcely been entered on, and it is perhaps not desirable, seeing how very small a portion of the Insects of the world have been critically examined, that much effort should be yet expended on an attempt of the kind. It must be admitted that the species of Insects should be obtained before they can be satisfactorily classified, and it is estimated^[112] that at least nine-tenths of the Insects of the world are still unknown to entomologists.

Geological Record.—Although Insects have a very long pedigree, it is as yet a very imperfect one. The remains of creatures that can be referred to the Class Insecta have been found, it is said, in Silurian strata; only one or two of these very early forms are at present known, and the information about them is by no means satisfactory; if Insects at all—as to which some doubt exists—they apparently belong to very different forms, though, like all the earliest fossil Insects, they are winged. In the strata of the Carboniferous epoch numerous Insects have been detected, in both Europe and North America. These earlier Insects are by Scudder called Palaeodictyoptera, and separated from the Insects around us on the ground that he considers there existed amongst these palaeozoic Insects no ordinal distinctions such as obtain in the existing forms, but that the primeval creatures formed a single group of generalised Hexapods. Brauer does not accept this view, considering that the earlier Insects can be referred to families existing at the present time and forming parts of the Orthoptera, Neuroptera, and Hemiptera. The discrepancy between these two authorities depends to a great extent on the different classifications of existing Insects that they start from; Scudder treating the wings as of primary importance, while Brauer assigns to them only a subordinate value. From the point of view taken in the present work Scudder's view appears to be in the main correct, though his expression as to the primary fossil Insects forming a single homogeneous group is erroneous. The Neuroptera, still in existence, certainly form a heterogeneous group, and it is clear that the Palaeozoic fossils represent a more diverse assemblage than the present Neuroptera do.^[113]

In the more recent rocks Insect remains become comparatively numerous, and in Mesozoic strata forms that can satisfactorily be referred to existing Orders are found, the Palaeodictyoptera of Goldenberg and Scudder having mostly disappeared; the Blattidae or cockroaches do not apparently present any great discontinuity between their Palaeozoic and Mesozoic forms. The Tertiary rocks afford us fairly satisfactory evidence to the effect that Insects were then more numerous in species than they are at the present day. At Florissant in Colorado the bed of an ancient lake has been discovered, and vast quantities of Insect remains have been found in it, the geographical conditions indicating that the creatures were not brought from a distance, but were the natural fauna of the locality; and if so we can only conclude that Insects must have been then more abundant in species than they are now.

Scudder has informed us^[114] that not only were Insects abundant in the Tertiaries, but that their remains indicate conditions of existence very similar to what we find around us. "Certain peculiarities of secondary sexual dimorphism accompanying special forms of communistic life, such as the neuters and workers in Hymenoptera and the soldiers among the Termitina, are also found, as would be expected, among the fossils, at least through the whole series of the Tertiaries. The same may be said of other sexual characteristics, such as the stridulating organs of the Orthoptera, and of peculiarities of oviposition, as seen in the huge egg-capsules of an extinct Sialid of the early Tertiaries. The viviparity of the ancient Aphides is suggested, according to Buckton, by the appearance of one of the specimens from the Oligocene of Florissant, while some of the more extraordinary forms of parasitism are indicated at a time equally remote by the occurrence in amber of the triungulin larva of Meloe, already alluded to, and of a characteristic strepsipterous Insect; not only, too, are the present tribes of gall-making Insects abundant in the Tertiaries, but their galls as well have been found."