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].
3. The small intestines (Intestina parva) are the portion of intestines next the stomach, and consist often of three distinct canals;—the first is supposed to be analogous to the duodenum; it is found only in the Coleopterous genera Silpha L. and Lampyris L., and is distinguished from the succeeding intestine by being perfectly smooth[455]. Next follows the thin intestine (Dünndarm), which in the above insects is wrinkled; it most commonly immediately follows the stomach. Sometimes it is wholly wanting, as in Agrion, the Hemiptera[456], &c. Ramdohr conjectures that it is not solely destined for conveying the excrement, but that probably some juices are separated in it from the food especially for the nutrition of the gall-vessels, as their principal convolutions are mostly near this intestine[457]; which perhaps may in some cases be regarded as analogous to the jejunum in vertebrate animals. The third pair of the small intestines, which perhaps represents the ileum, Ramdohr distinguishes by the name of club-shaped (Keulförmigen Darm[458]). It may generally be regarded as only a continuation of the former thickened at the end so as to resemble a club reversed. It is however sometimes separated from the thin intestine, as in Cerambyx moschatus[459].
4. The large intestines (Intestina magna) consist sometimes of two portions. The thick intestine (Dicken-darm), which may be regarded as a kind of cœcum, is found only in the larvæ of the Lamellicorn beetles, but never in the perfect insect. In shape it is oval and folded; whence it is thicker than the rest of the intestinal canal, and is constantly filled with excrement[460]. The second portion of these intestines is the rectum (Mastdarm), which terminates in the anal passage. This part is scarcely ever wanting, except when the insect evacuates no excrement, which is the case with the grubs of bees, wasps, and the antlion (Myrmeleon). In the imago of Telephorus, at least in T. fuscus, it is also obsolete[461]: in most cases, however, it is very distinct from the preceding intestine. Sometimes it consists of only one tunick composed of muscular fibres[462]. When the gullet is wide, the rectum is usually so likewise; but when it follows a club-shaped or thick intestine, it is narrow[463]. It generally may be termed short[464]. When wide, it often contains a great quantity of excrement, as the gullet does of undigested food; but when narrow, the excrement seldom remains long in it. This intestine also in a few cases has a lateral enlargement or cœcum (Blind-darm), being a continuation of the same skin; but perhaps this enlargement is really analogous to what Ramdohr calls the thick intestine, though in these cases he regards it as an appendage of the rectum[465].
I must now call your attention to the bile-vessels of insects. These, by Malpighi[466] and the earlier physiologists, who regarded them as a kind of lacteals, were denominated varicose vessels: but Cuvier—and his opinion after some hesitation has been adopted by Ramdohr—considers them as vessels for the secretion of bile, and as analogous to the liver of animals that have a circulation[467]. As the want of blood-vessels prevents insects from having any gland, the bile is produced with them, as all their other secretions, by slender vessels that float in their nutritive fluid, and from thence secrete the elements proper to form that important product, which usually tinges them with its own yellow hue; though in the Lamellicorns and Capricorns they are of an opaque white, and in the Dytisci of a deep brown colour[468]. Their bitter taste further proves that they contain the bile[469]. They are long, slender, filiform, tortuous or convoluted, and mostly simple vessels; sometimes gradually smaller toward the base[470], at others towards the apex[471]. In some, screw-shaped[472]: in one larva, with hemispherical elevations[473]: in the cockchafer, part of them are fringed on each side with an infinity of short, blind, minute, setiform tubes, while the rest are naked[474]; they are composed of a single, thin, transparent membrane, according to Ramdohr[475]; but Cuvier thinks their texture is spongy[476]. They appear to contain a number of small, irregular, dark granules, which float in a peculiar fluid, with which, however, they are not always filled throughout, nor are they constantly permeable from one end to the other. Thus in the meal-worm beetle (Tenebrio Molitor), the common trunk by which they are attached to the intestinal canal is composed of gelatinous granules[477]. The place of their insertion is generally a little below the pylorus, but in the common cockroach they are inserted into the stomach just above that part[478]. Usually each vessel opens singly into the intestinal canal, which the whole number surround at an equal distance from each other[479]. Sometimes, however, they are connected with it by a common tube in which they all unite, as in the asparagus-beetle (Lema Asparagi[480]), and the mole-cricket (Gryllotalpa vulgaris[481]); in the house-fly (Musca domestica), and other Muscidæ, each pair unites so as to form a single branch on each side of the canal previously to their insertion[482]; in the field-cricket (Gryllus campestris) they are all inserted in one spot[483]; and when numerous, they are generally attached singly though irregularly[484]. These vessels at their base do not open into the cavity of the intestinal canal, but merely into the space between its outer and inner tunicks, the last being constantly imperforated[485].
With regard to their apex, the bile-vessels are sometimes fixed singly or connectedly to the intestine merely by a few muscular fibres; for they do not enter it, their ends having no orifice. This structure is mostly to be met with in the Coleoptera[486]. In caterpillars, the tops of these vessels perforate the outer skin of the rectum, and proceeding in dense convolutions to the anus, become at last so fine that their terminations cannot be discovered[487]. In other cases, the extremities of a pair of these vessels unite so as to form a double one: this may be seen in those of Philonthus politus[488], and probably other rove-beetles: and lastly, in others the bile-vessels are free, hanging down by the intestinal canal, without being attached to it or to each other. This structure is constantly found in the Orthoptera and Hymenoptera Orders, &c.[489].
With regard to their number, the bile-vessels vary from two to upwards of one hundred and fifty, yet so that their whole amount is constantly the product of the number two,—at least as far as they have been counted: and even when those on one side are not alike, a similar variation takes place in the other, as may be seen in Galleruca Vitellinæ, where on each side are two long ones and one shorter[490]; the most usual numbers are, four—six—or many, that is, more than twenty—