Part 8

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.

I am, &c.

FOOTNOTES:

[348] _Genes._ ix. 4.

[349] _N. Dict. d'Hist. Nat._ xxx. 130.

[350] Cuv. _Anat. Comp._ iv. 167.

[351] Herold _Schmetterl._ 25. note *. VOL. III. p. 53.

[352] _N. Dict. d'Hist. Nat._ vii. 313. Cuv. _Anat. Comp._ iv. 411.

[353] _Ibid._ 419, 407.

[354] _Ibid._

[355] _Ibid._ 410.

[356] PLATE XXII. FIG. 15.

[357] Lyonet _Anat._ 105.

[358] _Ibid._ 425.

[359] _Ibid._ 105--.

[360] _De Bombyc._ 15--.

[361] Reaum. i. 160--.

[362] Cuv. _Anat. Comp._ iv. 418.

[363] Marcel de Serres _Mem. du Mus._ 1819. 69.

[364] Swamm. _Bibl. Nat._ _t._ xl. _f._ 4. _t._ xv. _f._ 4.

[365] _De Bombyc._ _t._ iii. _f._ 4.

[366] _Ubi supr._ 414.

[367] _Ibid._ 425--.

[368] _Ibid._ 419.

[369] _Ibid._ 412.

[370] Lyonet _Anat._ 413.

[371] Lyonet _Ibid._ 426. Cuv. _Anat. Comp._ iv. 419.

[372] Lyonet says (426), "au-delà de trois millions de fois plus petits qu'un grain de sable"!!

[373] _Ibid._

[374] His words are--"In silkworms I have clearly seen various small vessels spring from and approaching to the heart, which I have even filled with a coloured liquid. But whether they were veins or arteries I cannot yet affirm." i. 112. a. 176. a. According to Cuvier (_Anat. Comp._ iv. 418), but I cannot find the passage, Swammerdam also mentions having seen a red fluid issue from small vessels in grasshoppers.

[375] Reaum. v. 103.

[376] Bonnet ii. 309. Perhaps in both cases the alimentary canal was the organ seen.

[377] Reaum. iv. 171--.

[378] Lesser L. ii. 84. note.

[379] De Geer ii. 505--. vi. 287.

[380] _On the Microscope._ i. 130.

[381] _Ibid._

[382] _Sur le Vol des Ins._ 325--.

[383] Lyonet _Anat._ 427--.

[384] Cuv. _Anat. Comp._ iv. 418--.

[385] _Mem. du Mus._ 1819. 71.

[386] _N. Dict. d'Hist. Nat._ xvi. 208.

[387] Marcel de Serres, in his _Observations on the Dorsal Vessel of Insects_[388], endeavours to prove that the principal use of that vessel is the more perfect animalization of the chyle that, transuding through the pores of the intestinal canal, is imbibed by it. In insects, he observes, that undergo metamorphoses, in which the growth or development of parts is often very rapid, it is requisite that a considerable portion of the chyle should be in reserve for this purpose. On this account it is that the _Epiploon_ or adipose tissue is so abundant in larvæ to what it is in the perfect insect. That the importance also of this part to insects is proved by the circumstance, that all their interior parts communicate by fibrils with this tissue, and that probably their various organs derive the nutriment from it by their means. He then asks by which of the viscera is the fat elaborated, or by what means does the chyle which transudes from the intestinal canal pass to the state of fat? Facts seem to indicate, says he, that the function of the dorsal vessel is to pump up the chyle, and to cause it then to transude through the meshes of the adipose tissue, where it finishes by elaborating that mass of fat so abundant in larvæ and certain perfect insects, which are thus enabled to sustain the effects of a long fast. So that this vessel is only a _secretory_ organ, analogous to so many others that exist in insects; but the secretion which it has to produce is the most important of all, since the support of the vital powers depends upon it: it is, in effect, that vessel which completes the function of animalization, and which itself prepares the nutritive fluid[389]. He observes, amongst other reasons he brings to support his theory, that the colour of the fluid which it contains is always analogous to that of the adipose tissue that surrounds it, and that the colour of that tissue never changes without that of the fluid undergoing a corresponding alteration,--that when, as in many perfect insects, the quantity of fat diminishes, the dorsal vessel also diminishes in size, and that the same reagents which coagulate the fat, coagulate equally the fluid in the dorsal vessel, which seems to indicate an identity between them[390].

But there are circumstances that militate against this hypothesis. The analysis which Lyonet has given of the fluid contained in the dorsal vessel of the _Cossus_[391], seems to prove that it is more analogous to gum or varnish. He saw indeed a few globules, which appeared ten times as big as the others, which swam upon the water, but which he did not regard as component parts of the fluid, but as little drops of grease extravasated by dissection. The fluid of the vessel itself easily mixed with water, and appeared to sink in it to the bottom[392]. This proves that it is not of a fatty or oleaginous nature. But the strongest objection is stated by M. Carus, who judiciously observes[393], That it is contradictory to suppose that a canal should absorb or exude fluids by its parietes in a different form. Further experiments however seem necessary to ascertain the nature of the fluid and its object.

[388] _Mem. du Mus._ 1819.

[389] _Ibid._ 68--.

[390] _Ibid._ 69--.

[391] See above, p. 85.

[392] Lyonet _Anat._ 426--.

[393] _Introd. to Comp. Anat._ ii. 277. Engl. Trans.

[394] This seems some confirmation of Dr. Virey's opinion, that insects in their first states are still a kind of _fœtus_. See above, VOL. III. p. 61--.

[395] _Introd. to Comp. Anat._ ii. 393--. Engl. Trans.

[396] _Introd. to Comp. Anat._ ii. 395--. Engl. Trans.

[397] _Introd. to Comp. Anat._ ii. 396--. Engl. Trans.

[398] _Ibid._ 398.

[399] _Ibid._ 399.

[400] _Ibid._ 398.

[401] _Introd. to Comp. Anat._ ii. 399--. Engl. Trans.

[402] Reaum. i. 409, 643--. Malpigh. _De Bombyc._ 38.

[403] Lesser L. ii. 87 note *.

[404] _Ubi supra._

[405] Reaumur iv. 264.

[406] Ibid. 260--.

[407] Herold _Schmetterl_. 24.

[408] _Anat. Comp._ iv. 165.

[409] Marcel de Serres (p. 67.) speaks of this fluid as being, after it has transuded through the intestinal canal, a fluid in _repose_, which seems to indicate that it is perfectly _stagnant_; but when we consider that it is not only incessantly entering the body and making its way to every part, but is also, by means of the various secretory organs, constantly converted into new products, and so going out again in many cases, it will appear evident that it cannot be considered as a stagnant fluid, since there must be a constant though probably slow motion towards the points of absorption or imbibition.

[410] Dr. Kidd (_Philos. Trans._ 1825. 236.) did not find the abdominal viscera of the mole-cricket thus circumstanced, nor more lubricated than the intestines of the higher animals.

[411] Cuv. _Anat. Comp._ iv. 158. Herold _Schmetterl._ 28.

[412] _Sur le Vol des Ins._ c. iv. 88. note 1.

[413] _Anat._ 428.

[414] Treviranus _Arachnid._ 28. _t._ iii. _f._ 28, 29.

[415] _Ibid._ 29. _t._ iii. _f._ 30, 31.

[416] _N. Dict. d'Hist. Nat._ xxx. 420. Comp. Treviran. _Arachnid._ 10--.

[417] _Ibid._ 9--.

LETTER XL.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONTINUED._

DIGESTION.

"The immense Class of insects," says the immortal Cuvier, "in the structure of its alimentary canal exhibits as many variations as those of all the vertebrate animals together: there are not only the differences that strike us in going from family to family and from species to species; but one and the same individual has often a canal quite different, according as we examine it in its larva or imago state; and all these variations have relations very exact, often easily estimable, with the temporary or constant mode of life of the animals in which it is observable. Thus the voracious larvæ of the _Scarabæi_ and butterflies have intestines ten times as large as the winged and sober insects--if I may use such an expression--to which they give birth[418]."

In the natural families of these creatures, the same analogy takes place with respect to this part that is observable in the rest of the Animal Kingdom; the length and complication of the intestines are here, as in the other Classes, often an index of a less substantial kind of nutriment; while their shortness and slenderness indicate that the insect lives by prey[419].

In considering therefore the parts connected with the digestive functions of the insect world, it will not be amiss to have reference to their _food_, and their mode of taking it; but first it will be proper to state and define the parts of this important organ.

In general the alimentary canal[420] is composed of the same essential _tunicks_ as that of the vertebrate animals, consisting of an interior epidermis, a papillary and cellular tunick, and an exterior muscular one[421]. The first is usually tender, smooth, and transparent; but not always discoverable, perhaps on account of its tender substance[422]. Ramdohr does not notice the papillary and cellular tunicks; they are probably synonymous with what he denominates--the _flocky layer_ (_Die flockige lage_), and which he describes, when highly magnified, as appearing to consist of very minute globules or dark points, and as being of a cellular structure[423]. The _exterior_ tunick is thicker and stronger than the _interior_, and composed of muscular fibres, running either longitudinally, or transversely so as to form rings round the canal. This tunick mostly begins at the mouth, and goes to the anus, changing its conformation in different parts of the above intestine. Sometimes however it originates only at the beginning of the stomach[424]. With respect to its general disposition, that canal--in its relative length, in the size of its different parts, in the number and form of its dilatations, and particularly of its stomachs and its cœcums, and in the folds of its interior--exhibits variations altogether analogous to those of vertebrate animals, and which produce similar effects[425]. As to its _parts_, it may be considered as consisting of _two_ larger portions, between which the biliary or hepatic vessels form the point of separation. In the first, the most universal parts are the gullet and the stomach; and in the second, the small intestine and the large intestine[426].

1. The _gullet_ (_Œsophagus_[427]) is that portion of the intestinal canal which, receiving the food from the pharynx, or immediately from the mouth, conveys it to the stomach. Though it often ends just behind the _head_[428], it is usually continued through the _trunk_, and sometimes even extends into the middle of the _abdomen_[429]; it therefore seldom much exceeds in length half the body. It is constantly long when the head is connected with the trunk by a narrow canal--as in the _Hymenoptera_, _Neuroptera_, _Lepidoptera_, &c.; but is frequently short when these parts are more intimately united[430]. It often ends in a kind of sac analogous to the crop of birds. Under this head I must mention a part discovered by Ramdohr, which he calls the _food-bag_ (_Speisesack_), as he thinks, peculiar to _Diptera_[431]. From the mouth in these proceeds a narrow tube into the abdomen, where it expands into a blind sac having no connexion with the stomach; so that the fluid food, as blood, &c. stored in it, must be regurgitated into the mouth before it can pass into that organ[432]. Thus these animals, besides their stomach, have a _reservoir_ in which to store up their food; the product therefore of a single meal will require several days to digest it.

2. The _stomach_ (_Ventriculus_[433]) is that part of the intestinal canal immediately above the bile-vessels, which receives the food from the gullet for digestion, and transmits it when digested to the lower intestines[434]. By its admixture with the gastric juice, the food acquires in the stomach a quite different colour from what it had in the gullet. In herbivorous _insects_ it contains no acid, but, like the gastric juice of herbivorous _quadrupeds_, is of an alkaline nature[435]. The chyle is forced through this organ, probably in part by the pressure of the muscular fibres during the peristaltic motion; and being pressed through the _inner_ skin, is first collected in the intermediate cellular part, and ultimately forced through the _outer_ skin[436]. At its posterior end it terminates in the _pylorus_, a fleshy ring or sphincter formed of annular muscular fibres[437]. The stomach often consists of two or more successive divisions, which are separated from each other, and are often of an entirely different conformation and shape[438]. In the _Orthoptera_, Predaceous _Coleoptera_, and several other insects, an organ of this kind precedes the ordinary stomach, which from its structure Cuvier denominates a second stomach or _gizzard_[439]; Posselt improperly calls it _Cardia_[440]; and by Ramdohr it is named the _plaited-stomach_ (_Falten-magen_[441]). It is a short fleshy part consisting of two skins, placed above the opening of the stomach, and perhaps rather belongs to the gullet. The _inner_ skin is formed into longitudinal folds, and sometimes armed with horns, teeth, or bristles. Its cavity is very small and compressed, so as to admit only small masses of food, and yet present them to a wide surface for the action of the teeth or bristles;--in this stomach therefore, as in the gizzard of birds, to which it seems clearly analogous[442], the food is more effectually comminuted and rendered fit for digestion. The muscles, by which its action upon the food is supported, in some species amount to many thousands[443]. Rudiments of a gizzard are sometimes found concealed in the gullet of many insects[444]. The idea of Swammerdam, Cuvier, &c. that grasshoppers and other insects that have this kind of stomach, chew the cud[445], Ramdohr affirms is entirely erroneous[446]. Besides its divisions, the stomach has other _appendages_ that require notice. In most _Orthoptera_, a pair or more of blind intestines or _cœca_ may be found at the point of union of the gizzard with the stomach[447], which have been regarded as forming a _third_ stomach: they also begin the stomach in the louse[448]; they form a coronet round the apex of that organ, in the grub of the cockchafer[449]; and in that of the rose-beetle, there is one at the apex, one in the middle, and a third at the base[450]. Besides these appendages, which are formed of the skin of the stomach, there are others that are not so. In the Predaceous and some other beetles, the whole external surface of this organ is covered with small blind appendages opening into the space between its two skins, which cause it to resemble a shaggy cloth; these Ramdohr calls _shags_ (_zotte_[451]), and Cuvier, _hairs_[452] (_villi_). These appendages the latter author seems to regard as organs that secrete the gastric juice and render it to the stomach[453]; but the former thinks their use uncertain[454].