| Two bile-vessels are found in | the larva of Cetonia aurata[491]. |
| Four | most Coleoptera, Diptera, and Hemiptera[492]. |
| Six | Lepidoptera, some Coleoptera[493], &c. |
| Eight | Myrmeleon, Hemerobius[494]. |
| Fourteen | Formica rufa[495]. |
| Twenty | larva of Clavellaria Amerinæ[496]. |
| Many | Libellulina, Orthoptera, and Hymenoptera[497]. |
The bile-vessels vary considerably in length: in many cases where they are free they are short[498]; they are often very long, and perhaps those that are fixed may be generally stated as the longest. In the Lamellicorn beetles they are remarkable for their great length[499].
Having given you this general account of the intestinal canal and its parts and appendages, I shall now state some of the peculiarities that in this respect distinguish particular tribes and families.
The Coleoptera alone, exhibit as many variations in the structure of the alimentary tube as all the other Orders of insects together:—to particularize these would occupy too large a portion of this letter, I shall therefore only notice a few of the most remarkable. In general they may be stated as having universally a stomach, a small intestine and rectum, and not more than three pairs of fixed or united bile-vessels. In the Predaceous beetles, the gullet mostly widens at the base into a considerable crop, followed by a gizzard, a shaggy stomach, and two pairs of united bile-vessels. The whole alimentary canal in these, is never less than double, and sometimes treble the length of the body[500]. In the carnivorous beetles, at least the Staphylinidæ and Silphidæ, there is little or no crop, and the gizzard is hidden: in the former, the whole length of the intestinal canal is not twice, while in the latter it is more than four times that of the body[501]. In these also the intermediate portion of the large intestine is singularly annulated[502]. In the Petalocera the stomach is usually longer than all the rest of the intestines together, and often convoluted: in the cockchafer the whole intestinal canal is nearly five times the length of the body, four parts of which is occupied by the stomach[503]. In the grub the canal scarcely exceeds the length of the animal[504]. In Lampyris the stomach exhibits a remarkable appearance, having on each side a series of spherical folds or vesicles[505]. Have these any thing to do with the secretion of its phosphoric matter? Tenebrio has a gizzard armed internally with calluses, and a shaggy stomach, and Blaps does not differ materially; their entire canal is more than twice the length of the body[506]. In the vesicatory beetles (Cantharis, Meloe, &c.) there is no gizzard, and the canal is less than twice the length of the body[507]. Little is known with regard to the alimentary canal of the beetles distinguished by a rostrum (Rhyncophora). In the only two that appear to have been examined, Rhynchites Betuleti and Cryptorhynchus Lapathi, that canal is moderately long, the stomach partially shaggy, and the small intestine inversely claviform; but in other respects they differ materially[508]. In the former there is no crop or gizzard, the stomach is fringed on each side, except at its upper extremity, with a series of small cœca or shags, and there are three pairs of bile-vessels[509]; while in the latter the gullet is dilated into a crop which includes a gizzard in which the skill of a Divine artist is singularly conspicuous:—though so minute as scarcely to exceed a large pin's head in size, it is stated to be armed internally with more than 400 pairs of teeth, moved by an infinitely greater number of muscles[510]. A transverse section of this gizzard represents two concentric stars, with nine rays each[511]: the object of this structure is, the comminution of the timber which this beetle has to perforate and probably devour[512]. The stomach is very slender, but dilates in the middle into a spherical vesicle[513], and there are only two pairs of bile-vessels[514]. In the Capricorn beetles, the part we are considering varies much: in general we may observe that it is more than double the length of the body, that the stomach is long and slender, and usually naked, that the gullet terminates in a crop without a distinct gizzard, and that there are three pairs of bile-vessels[515]. In the Herbivorous beetles (Chrysomela, Cassida, &c.) the canal is more than double the length of the body, and in some much longer[516], the stomach is long, and commonly naked; but in Chrysomela violacea it is covered with hemispherical prominences[517], and in Chrysomela Populi it is shaggy[518]; in the insect last named and Galleruca Vitellinæ the rectum consists of two pieces[519]. In this tribe the intestines of the larva resemble those of the perfect insect[520].
In the Orthoptera the alimentary canal, which continues the same in every state, is short, or only moderately long; the gullet has one or two lateral pouches or crops[521], and terminates in a gizzard of curious construction, with singular folds and teeth[522]; then follows a short stomach, usually with a pair or more of cœca at its upper extremity[523]; the lower intestines are not distinct, and the bile-vessels numerous, short and free[524].
In the Neuroptera, many of the genera are distinguished by the remarkable length of the gullet, and by the lower intestines forming one short piece[525]. In the Libellulina the bile-vessels are numerous, short, and free, as in the Orthoptera[526]. In Hemerobius and Myrmeleon there is a gizzard[527], and just above it a cœcum, in the former very remarkable, is connected with the gullet[528].
The Hymenoptera appear all to be distinguished by a long slender gullet, terminating in a dilated crop forming the honey-bag; their stomach is variable, their small intestine slender, and the rectum dilated;—their bile-vessels, like those of the two preceding Orders, are numerous, short, and free[529]. In the ants and ichneumons there is an approach to a gizzard[530]. In the wasp and humble-bee the stomach is very long, with muscular rings surrounding it[531]. In this Order the larvæ at first have no lower intestines and void no excrement[532], but as they approach to the pupa state one begins to appear[533].
The next insects whose alimentary canal we are to consider, are those which, taking their food by suction, have no occasion for masticating organs: this may in part be predicated of the preceding Order, in which most of the tribes in their perfect state imbibe fluid food, and use the ordinary organs of mastication principally in operations connected with their economy; and their crop, in which the honey in many is stored up for regurgitation, may be regarded in some degree as analogous to the food-bag of the Diptera and other suctorious insects.
The two sections of the Hemiptera Order differ widely in the canal we are considering, and I shall therefore give a separate account of each. In the Heteropterous section, appended to the gullet by a long convoluted capillary tube, besides the usual saliva-reservoirs there is often a double vessel, which Ramdohr regards as discharging the same function, but which in many respects seems rather analogous to the food-reservoir of the Diptera[534]. As I have had no opportunity of examining this vessel, I shall content myself with stating this idea, and describe the vessel more fully hereafter. The gullet, in these, usually terminates in an ample crop consisting of many folds[535], followed by a long, slender, cylindrical tube, dilated at its base into a spherical tumour; these two may be said to form the first stomach: to this succeeds a second[536], which Ramdohr denominates the bug-stomach (Wanzen-magen), which varies in its figure, and in Pentatoma consists of four demi-tubes, so as to form a quadrangular canal[537]. In the Homopterous section of this Order Ramdohr seems to have examined but few; Chermes however and Aphis exhibit one remarkable feature; they have no bile-vessels, at least he could discover no trace of these organs[538]. Their intestinal canal is very simple, their stomach very long, widest above, and somewhat convoluted, with a very slender gullet[539]. In Cercopis spumaria the structure is more complex, and extremely singular. It has two or rather three stomachs; the two first of a horny substance, and the last a slender somewhat convoluted membranous tube, which becoming reversed, is attached by what should be deemed its lower extremity to the first stomach, from the other side of which emerge the lower intestines, terminating in a thick pear-shaped rectum. At the same point of the first stomach the four bile-vessels are attached, they grow gradually thicker for about a third of their length, when they become twisted like a cord, and taper towards the rectum, to which also they are attached[540]. From this structure it should seem that the food has to pass twice through the first stomach, before the process of digestion is complete, and it is rejected at the anus.