Volume 4 — LETTER XXXIX.

INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED.

CIRCULATION.

We learn from the highest authority, that the blood is the life of the animal[348]: every object of creation, therefore, that is gifted with animal life, we may conclude, in some sense, has blood, which in this large sense may be defined—The fluid that visits and nourishes every part of a living body[349]. But the Great Author of nature has varied the machinery by which this nutritive fluid is formed and distributed, gradually proceeding from the most simple to the most complex structure; in which he seems to have seen it fit to invert the process observable in the systems of sensation and respiration, where the ascent is from the most complex, to the most simple structure. In the lowest members of the animal creation, the blood seems the portion they imbibe of the fluid medium in which they reside, which when chylified, distributes new molecules to all parts of their frame[350]. In others, as in insects, it is formed by the chyle that transpires through the intestinal canal into the general cavity of the body, where it receives oxygen from the air-vessels, and is fitted for nutrition[351]. In these animals it is accompanied by a long dorsal vessel, the first step towards a heart, which alternately contracts and dilates with an irregular systole and diastole, but appears to have no vascular system connected with it, though in their preparatory states it has an extra-vascular circulation which ceases in the perfect insect. Again: in others, as the Tubicoles, Annelida, &c., a real circulation has been discovered; that is to say, a system of veins and arteries, but unaccompanied by a muscular heart[352]. In the Arachnida and Branchiopod Crustacea the long dorsal vessel is also found; but in these it is connected with an arterial and venous system, which receives, distributes, and returns the blood[353]. It has therefore now become a true heart, and there is a regular circulation; and in the Decapod Crustacea the dorsal vessel is contracted into an oval form, and placed nearly in the centre of the trunk[354]. In the great majority of invertebrate animals the blood is white, but in the Annelida, to which Class the common dew-worm belongs, a curious anomaly takes place—for it is red[355]. Thus a gradual ascent is made to the circulating system of the vertebrate and red-blooded animals. In all, however, the blood is the principal instrument of nutrition and accretion; and is on that account properly so denominated, though not connected with a circulating system.

Having given you this general outline of the means by which the blood is distributed in the different Classes of animals, I shall now confine myself to the case of insects and Arachnida, beginning with the former.

I. If you examine attentively the back of any smooth caterpillar with a transparent skin, you will perceive in that part an evident pulsation, as though a fluid were pushed at regular intervals towards the head, along a narrow tube which seems to run the whole length of the body. Accurate dissections have proved that this appearance is real, that there is actually present in the back of most insects, placed immediately under the skin and furnished with numerous air-vessels, a longitudinal vessel[356] originating in the head near the mouth[357], running parallel with the alimentary canal nearly to the anus, containing a fluid which is propelled in regular pulsations of from 20 to 100 per minute, more or less as the weather is colder or warmer[358], causing a sensible alternate systole and diastole from the anal extremity towards the head. In the Cossus these pulses were observed by Lyonet to begin in the eleventh segment, from which they passed from segment to segment, till they arrived at the fourth, where they terminated[359]. This vessel is what Malpighi, who first discovered it, termed a heart, or rather series of hearts[360]; but which Reaumur, who injected it, regarded as a simple artery without striking contractions[361]: but to steer clear of any hypothesis, I shall merely call it the dorsal vessel (Pseudo-cardia). When carefully taken out of the body it is found to be a membranous tube, appearing to be closed at each end[362], in many larvæ of equal diameter every where, but in perfect insects usually widest at the anal extremity[363], and attenuated into a very slender filament towards the head. In some insects, however, as in the larva of the chamæleon-fly (Stratyomis Chamæleon), it is attenuated at both ends, and in the Ephemera is alternately constricted and dilated as Malpighi describes that of the silkworm[364], a dilated portion belonging to each segment[365]. In the Cossus, and probably others, after the third segment, it is furnished with nine pair, the three posterior pair being the largest, of triangular transverse bundles of muscular fibres, which Lyonet denominates its wings[366], the action of which produces its systole and diastole, and their propagation from the tail towards the head[367]. Under the last pair of these wings it is strengthened by a large number of circular muscular fibres[368]. I have stated it as appearing to be closed at each extremity, because Cuvier and most writers have so regarded it, and probably it is so closed in the perfect insect; but from Lyonet's words it should seem that, in the larva of the Cossus, he considered it as open and expanded at its anterior end[369]. He seems also to suspect, that, by means of what he calls the frontal ganglions, a fluid is derived from the dorsal vessel to the spinal marrow. He likewise describes a large nerve as passing through it and becoming recurrent[370]. Carus, as we shall soon see, has also proved that this tube is not closed in larvæ.

The fluid which this vessel contains is very abundant; in the animal it appears colourless and transparent like water, but when collected in drops it becomes more or less yellow, and even orange[371]. Examined under the microscope it appears filled with a prodigious number of transparent globules, of incredible minuteness[372]. When mixed with water, which it does readily, its globules lose all their transparency, and coagulate into small clammy masses. After evaporation it becomes hard, and cracks like gum, as blood does also. This gummy substance is so abundant, that the fluid contained in the dorsal vessel of the caterpillar of the Cossus yields a mass of it of the size of a grey pea[373].

From the situation of this dorsal vessel, which is precisely the same with that of the heart in Arachnida and the Branchiopod Crustacea, and from the systole and diastole which keep its fluid contents in constant motion, who can wonder that the physiologists who first discovered it, reasoning analogically, maintained that it was a true heart? But modern comparative anatomists, and those of the highest name, from the absence of a vascular system for a circulation, have contended that it is not a true heart, but an organ appropriated to other purposes: a third hypothesis, and intermediate between these two, has very recently been promulgated, that the organ in question, namely, is a real heart, and in the preparatory states of insects, the centre of a real circulation, which, in the imago state, ceases with the full development of the wings; but that this circulation is extravascular, or without peculiar vessels analogous to veins and arteries.

I shall now enlarge a little upon each of these hypotheses, beginning with the first or original one.

No one will deny that the argument from analogy is strongly in favour of this: I need not therefore dwell upon it, but proceed to others. Swammerdam, to whose exactness in observing, and scrupulous accuracy, every reader of his immortal work will bear testimony, expressly asserts that he has seen vessels issuing from the dorsal vessel in the silkworm, and even succeeded in injecting them with a coloured fluid[374]. Now it seems extremely improbable that so practised and expert an anatomist should have been deceived, especially upon a point which would naturally excite his most earnest and undivided attention. Without this recorded experiment, perhaps, it might be thought, though this was very unlikely, that he had mistaken bronchiæ for veins and arteries: but how could they have been injected from the supposed heart? Another great physiologist, Reaumur, in the caterpillar of the saw-fly of the rose (Hylotoma Rosæ) observed, besides the dorsal vessel, a ventral one of similar form, in which also was a pulsation, but slower than that of the other. This he supposes may be the principal trunk of the veins[375]. Bonnet thought he discovered a similar vessel in a large caterpillar, but with all his attention could perceive no motion in it[376]. Reaumur also fancied he perceived in the grub of Musca vomitoria, in which he in vain looked for the dorsal vessel, a fleshy part which exhibited alternate pulsations; and when with a pair of scissors he made a lateral incision in the insect, amongst other parts that came out, there was one that had movements of contraction and dilatation for several minutes,—this experiment was repeated with the same result upon several grubs[377]. De Geer, whose love of truth and accuracy no one will call in question, saw the appearance of blood-vessels in the leg of the larva of a Phryganea L. (as Lyonet did in those of a flea[378]); and in the transparent thigh of Ornithomia avicularia he discovered a pulse like that of an artery[379]. Baker, whose only object was to record what he saw, speaks of the current of the blood being remarkably visible in the legs of some small bugs[380]: what he meant by that term is uncertain, but they could not be spiders, which he had just distinguished. This author has likewise seen a green fluid passing through the vessels of the wings of grasshoppers[381]; and M. Chabrier is of opinion that insects possess the power of propelling a fluid into the nervures of their wings and withdrawing it at pleasure, as they are elevated or depressed[382]; but this last fact may be independent of a circulation.

But though these arguments, which I have stated in their full force, appear strong, and at first sight conclusive, those which may be urged for the more modern opinion—that no circulation exists in insects, properly so called,—appear to have still greater weight. Lyonet, whose piercing eye and skilful hand traced the course of so many hundred nerves and bronchiæ long after they became invisible to the unassisted eye, and which were a thousand times smaller than the principal blood-vessels, opening into so large an organ as the supposed heart of insects, might be expected to be, could never discover any thing like them. His most painful researches, and repeated attempts to inject them with coloured liquors, were unable to detect the most minute opening in the dorsal vessel, or the slightest trace of any artery or vein proceeding from or communicating with it[383]. And Cuvier, whose unrivalled skill in Comparative Anatomy peculiarly qualified him for the investigation, repeated these inquiries, and tried all the known modes of injection, with equal want of success; and is thus led to the conclusion, that insects have no circulation, that their dorsal vessel is no heart, and therefore ought not to be called by that name: that it is rather a secretory vessel, like many others of that kind in those animals. As to the nature of the fluid that it secretes, and its use, he thinks it impossible, from our present information on the subject, to form any satisfactory conclusion[384]. Marcel de Serres informs us—which further seems to prove that it can be no real heart—that this vessel may be totally removed without causing the immediate death of the insect[385]. This opinion receives additional confirmation from the mode in which respiration is performed in insects. In those animals that have a circulation, this takes place by means of lungs or gills;—thus we find, even in the Crustacea and Arachnida so nearly related to insects, that the organs of this function are true gills; whereas in insects, though in some of their states their respiratory tubes are branchiform, yet they are not gills, and the respiration is by tubes and spiracles. And these tubes, as you have seen, are so numerous and so infinitely ramified and dispersed, as to occupy the place of arteries and veins, and to imitate their distribution,—and thus to oxygenate what may be deemed the real analogue of the blood, which bathes every internal part of the body of an insect. Those animals likewise that have a circulation are furnished with a liver, as is the case with the Arachnida and even many aggregate animals that have a heart; but in insects there are only hepatic ducts. M. Cuvier has also proved that the conglomerate glands, which exist in all animals that have a heart and blood-vessels, do not exist in insects, in which they are replaced by long slender secretory tubes, which without being united float in the interior of the body: from this circumstance, he is led to conclude that their nutrition is by imbibition or immediate absorption, as in the Polypi and other zoophytes, the chyle transpiring through the alimentary canal, and running uniformly to all parts of the body[386].

These arguments appear so satisfactory, that Physiologists in general seem to have been convinced by them that no circulation, at any time, takes place in insects, and that their supposed heart is merely a secretory vessel, though of what kind they were at a loss to conjecture[387]. But, convincing as they seem, they appear to have been founded in error, and on the idea that a circulation, as well as a heart, necessarily implies a vascular system consisting of veins and arteries; for by the recent discoveries of M. Carus, it has been satisfactorily proved that insects in their preparatory states, have an extravascular circulation, the arterial and venose currents not being confined by parietes. The observations upon which M. Carus' hypothesis is founded, were made in the Autumn of 1826; and an abstract of their results presented to the Union of German Naturalists and Physicians, which then held its meeting at Dresden, many of the members of which, as MM. Oken, Husche, Heyne, Purkinje, Otto, Weber, and Müller, had ocular proofs of the reality of the phenomena.

His first observations were made on the larva of Agrion Puella, which swims by means of three vertical laminæ attached to the tail; which, when the wings first appear as rudiments, begin to be exsiccated and are finally detached. Each of these laminæ, in its natural vertical position, presents an inferior abdominal and a superior dorsal edge, has two tracheæ running along its centre with ramifying bronchiæ, and consists of granular substance contained between two strata of the external integuments. A current of blood-globules enters each lamina somewhat nearer to its abdominal than to its dorsal edge, and running through the greater part of its length suddenly turns and bends its course back towards the body, somewhat nearer to the dorsal than to the abdominal margin of the lamina. The channel thus formed in the midst of the granular substance is perfectly transparent, except where it is occupied by the blood-globules, or crossed by the bronchiæ. The parietes of the channel are not strictly defined, nor formed by any thing like the coats of a vessel, the blood circulating through the granular Parenchyma; a circumstance however which is not peculiar to this case, but also occurs generally in the first states of the circulation, as it presents itself for instance in the embryo of Fishes, and in the figura venosa of the incubated egg[394]. The blood-globules are elongated like a grain of wheat, considerably larger than those of the human blood, and float in a fluid which is invisible because of its transparency, but the existence of which is proved by the variations in the position of the globules in the current, sometimes following its direction, at others crossing it transversely, or more or less obliquely.

When the animal is vigorous, the current is uninterrupted, although its velocity is accelerated at regular intervals; and that not only in the excurrent (arterial), but also in the recurrent (venous) part of its course through the lamina. When the animal becomes exhausted, or the laminæ exsiccated, the circulation is interrupted, and in the same manner, as under the same circumstances, in the larvæ of frogs and lizards; the disturbance displaying itself not merely by a cessation of the process, but also by retrograde movements of the currents, or by oscillatory motions of the blood-globules.

In proportion as the wings are developed, the circulation in the laminæ diminishes, and ultimately ceases, preparatory to the detachment of the laminæ themselves. At the same time, however, it presents itself under a new form in the wings. In these the excurrent or arterial stream takes its course along the inner margin of the wing, and the recurrent or venous returning along the outer; whilst, occasionally, other transverse currents take their course through the net-work of the wing from its inner to its outer margin. As the wings are further developed, the circulation in them, like that in the caudal laminæ, gradually becomes weaker and ultimately ceases[395].

The next observations were made on the transparent larva of a neuropterous insect (probably a Semblis or Sialis), in which the pulsations of the dorsal vessel were distinctly seen at its posterior extremity, from which they were propagated towards the anterior; these two divisions of that vessel appearing to bear to each other the relation of a heart and aorta. There were no traces of other vessels, though regular and rapid currents of blood-globules, exterior to the tracheæ, proceeded from the head towards the posterior extremity of the body, where each of these currents entered the heart, which again propelled its contents with accelerated velocity through the anterior part of the dorsal vessel towards the head. The lateral currents also were accelerated upon each contraction of the heart, proving that they must communicate with the dorsal vessel at the anterior part of the body, though the opacity of the head rendered it impossible to ascertain the mode of anastomosis. An excurrent and returning current were also traced to each of the legs[396]. But the phenomena of the circulation was most distinctly visible in the larva of Ephemera vulgata, even more distinctly than it is possible to trace it in the larvæ of frogs and newts. In this animal the circulation, with the help of the microscope, is at once visible in the three last segments of the body; and with a little attention is discoverable not only in the three terminal caudulæ, and in the upper joints of the legs, but also in the head, and particularly the roots of the antennæ. In the posterior part of the body there are on each side two currents of blood, not bounded by parietes, situate on each side of the intestinal canal, the inner one being the most considerable. The external one communicates with the internal by several intermediate branches; from this probably the streams are detached, which in the form of loops are seen at the upper joints of the legs, though it is not possible precisely to ascertain this, nor even whether these lateral currents continue distinct in the thorax, which probably they do. At the ninth abdominal segment these currents which flow posteriorly from the head, change their direction, and are inflected so as to enter the pulsating heart, from which the current again flows towards the head. Before they enter the heart they give off three streams, one for each of the three caudulæ. The currents in these caudulæ present the phenomena of the circulation with peculiar distinctness, and are particularly remarkable from the circumstance, that the excurrent and recurrent streams, though closely approximated without any visible separation, flow without disturbing each other. The excurrent stream is accelerated in correspondence with the pulsations of the heart; the recurrent on the contrary being always somewhat more sluggish, and the first to stagnate and cease when the strength of the animal is impaired. In the anterior part of the head currents can be discovered, forming loops like those of the legs, at the roots of the antennæ; each current proceeding from the cranial surface, and in returning taking its course towards the region of the larynx[397].

M. Carus has likewise observed currents of blood in the larvæ of water-beetles (Hydrophilus and Dytiscus)[398]; but at present he appears to have detected it in no terrestrial larva. Whether this is occasioned by their opacity, or it exists only in the ovum, as he seems to suspect[399], must be left for determination to future observers; it is scarcely probable, however, that the larvæ of Dytisci and Hydrophili should differ from other Coleoptera in their circulation.

The endeavours of M. Carus to discover any proofs of a circulation in insects in their last state, except in the wings at their first development, were without success[400]. He observes that the fact of the currents of fluids in larvæ not being defined by vascular parietes, enables us to comprehend the rapidity and facility with which the traces of the circulation are lost in the perfect insect. On the other hand, that the existence of a circulation at one period, and its cessation at another, elucidate many circumstances connected with the physiology of these animals: for instance, the contrast between the rapid growth and transformations of the larvæ, and the stationary existence of the imago, &c. Lastly he remarks, that the phenomena of this circulation do not throw any light on the obscure subject of the mode of nutrition in perfect insects; which therefore must still be supposed to be effected according to the idea of Cuvier, without the intervention of vessels[401].

Whatever be the functions of the dorsal vessel, this seems the most proper place to state to you what further is known respecting it. Its construction is nearly alike in insects in all their states, except that in the imago it is shorter and narrower. Reaumur has affirmed, and before him Malpighi made a similar observation, that in chrysalises newly disclosed from the larva, and yet transparent, the motion of the included fluid is the reverse of what it has been in that state, it being propelled from the head to the tail, which he found to be the case also in the imago[402]. If this be true, and there is no reason to doubt his accuracy, when they are more advanced, it resumes its old course, as Lyonet observed, from the tail to the head[403]. But probably it is not always uniformly in the same direction, since Malpighi states that a very slight cause will change its course, and that the pulsations differ in quickness in different portions of the heart[404]. If its course were really always the same, and in one direction, without any reflux, it would seem to follow that the fluid must be absorbed at one end, and, if there was no outlet, transpire at the other, which would be a kind of circulation. In Syrphus Pyrastri and other aphidivorous flies, this dorsal vessel, instead of the usual form which it had in the larva, assumes a very peculiar appearance. If, taking one of these flies by the head and wings and holding it up to the light, you survey under a lens the base of the lower part of its abdomen, you will see through its transparent skin, which exactly forms such a window as physicians have sometimes wished for in order to view the interior of their patients, a flask-shaped vessel having its long end directed towards the trunk, in which there is a manifest pulsation and transmission of some fluid. This vessel extends in length from the junction of the trunk with the abdomen to about the termination of the second segment. The included fluid does not run in the dorsal vessel in a regular course, but is propelled at intervals by drops, as if from a syringe, first from the wide end towards the trunk, and then in the contrary direction, forming a very interesting and agreeable spectacle. One circumstance led Reaumur to conjecture that the neck of this vessel, which he at first regarded as simple, is in fact composed of two or more approximated tubes, and that the blood is conveyed forward by the outward ones, and backward by the intermediate one[405]: he even thinks that he saw a kind of secondary heart, at the extremity next the trunk, for the purpose of causing the reflux. This illustrious author observed the above remarkable structure not only in the Syrphi, but in many of their affinities, and thinks that it is also widely diffused amongst the Muscidæ[406].

I must now say something upon what I conceive to be the real blood of insects; for I think no one will object to that name being given to their nutritive fluid, especially in the larva, though it does not circulate by means of a vascular system. The chyle that is produced in the intestines of animals from the food, is that fluid substance from which their blood is formed: in insects it is not absorbed by the lacteals, but transpires through the pores of the intestinal canal into the general cavity of the body, where, being exposed to the influence of the oxygen in the air-vessels, it becomes, though retaining its colour, a different fluid from what it was before, and analogous to blood in its use and office[407]; only that in these animals, as Cuvier has observed, at least in their perfect state, the blood, for want of a circulating system, not being able to seek the air, the air goes to seek the blood[408]. The dispersion of this fluid appears to be universal, so that all the parts and organs contain it in a greater or less degree[409]. In many insects, if you break only an antenna or a leg, a drop of fluid flows out at the wound. In larvæ, the fluid which bathes[410], or visits, all the internal parts and organs is not only sufficient for their nutriment, but a large quantity of seemingly superfluous blood remains that is not wanted for this purpose. This is expended in the production of the caul or epiploon (Corps graisseux Reaum.), which laps over and defends all the viscera of the animal, and goes principally to the formation of the imago[411]. I have said that Cuvier conceives nutrition in insects to take place by imbibition or immediate absorption; that is, I suppose, the different parts and organs thus constantly bathed in the blood, imbibe from it the particles necessary for their constant accretion. M. Chabrier seems to think that it is the compression and dilatation of the trunk that duly distributes the nutritive fluid[412]; Lyonet compares the nutrition of insects by their fibres from this fluid, when formed into the corps graisseux, to that of plants that draw their support by their roots from the earth[413]. Much obscurity, however, at present rests upon this subject—much for future investigation to explore; but in all the works of the Most High there is always something inscrutable, something beyond the reach of our senses and faculties, which teaches us humbly to adore his infinite perfections.

II. The circulation of the Arachnida is next to be considered; and the term applied to these becomes strictly proper. Two great tribes, in our view of the subject, constitute this Class,—the spiders (Araneidea) and scorpions (Scorpionidea): I shall give you some account of the circulating vessels of each.—In spiders, the heart in general is a long dorsal vessel as in insects, but supposed to be confined to the abdomen, growing slenderer towards each extremity, particularly the anal. In some also, as in Aranea domestica, like that of insects, it has lateral muscular appendages; but in others, as in Clubiona atrox, it is without them[414]. It exhibits a pair of vessels that appear to connect with the gills, by which the oxygenation of the blood takes place, and a number of others that ramify minutely and are lost in the analogue of the epiploon, supposed to be their liver[415]. Whether these last are to be regarded merely as veins, has not been ascertained; they seem rather to convey the blood outwards, than to return it back to the heart: but this question must be left for future investigation. I may observe, however, that though the heart of the spider has been traced only in the abdomen, it may probably extend into the trunk.

The heart of the scorpion has been examined both by Treviranus and Marcel de Serres; but I shall principally confine myself to the description of the latter, as the most clear and intelligible. The heart, then, of these animals is elongated, almost cylindrical, but attenuated at each end; it is extended from the head to the extremity of the tail, and appears to have four pairs of lateral muscles. On each side are four pairs of principal vessels, which go to the pulmonary pouches, and there ramify. These may be assimilated to veins. Besides these, there are four other vessels that cross them, forming with them an acute angle, and which, with four branches of smaller size, receive the blood from the pulmonary pouches, and distribute it to the different parts of the body,—these are the arteries. Before it enters the tail, the heart throws out two vascular branches which do not go to the gills, but distributing the blood to different parts, ought to be considered as arteries[416]. Treviranus mentions bunches of reticulated vessels, concerning the use and origin of which he seems uncertain[417]; but as they approach the gills they are probably the branching extremities of what M. de Serres considers as the veins.

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