The epithelial folds of the larvæ of dragon-flies serve as organs of respiration, the water being admitted into this cavity, and when forcibly expelled serving to propel the creature forward. Paired and single anal glands (repugnatorial) enter the rectum of certain Coleoptera (Figs. 302, l; 317, s; 320, e).
The vent (anus).—The external opening of the rectum is situated in the end of the body, in the vestigial 10th or 11th abdominal segment, and is more or less eversible. It is protected above in caterpillars, and other insects with 10 free abdominal segments, by the suranal plate. It is bounded on the sides by the paranal lobes, while beneath is the infra-anal lobe.
The anus is wanting in certain insects, and where this is the case the hind-gut, owing to a retention of the embryonic condition, is usually separated from the mid-intestine. (See p. 300.)
Fig. 321.—Enteric canal of Psyllopsis fraxinicola: œ, œsophagus; md, mid-intestine; ed, hind-intestine; vm, urinary vessels; s, the coil formed by the hind-intestine and the most anterior part of the mid-intestine.—After Witlaczil, from Lang.
Some remarkable features of the digestive canal in hemipterous insects are noteworthy. In the Coccidæ, according to Mark, the anterior end of the long mid-intestine forms, with the hinder end of the œsophagus, a small loop, whose posterior end is firmly grown to the wall of the rectum, and forms a cup-like invagination of the latter. Then the rest of the tube-like stomach turns sidewise and forms a large loop, which turns back on itself and occupies a large part of the body-cavity. This loop receives on the anterior end, near the œsophagus, the two urinary vessels, and forms just below the opening into the rectum a short cæcum.
In other homopterous genera (Psyllidæ and some Cicadidæ) Witlaczil describes nearly the same peculiarity, the mid-gut and part of the intestine forming a loop growing together for a certain distance and winding round each other (Fig. 321).
Histology of the digestive canal.—In all the divisions of the digestive canal of insects the succession of the cellular layers composing it is the same: 1st, a cuticula; 2d, an epithelium; 3d, connective tissue; 4th, muscular tissue. In the locust, the first division of the canal (fore-gut), there are two muscular coats, an internal longitudinal and an external circular coat; the fibres are all striated. The lining epithelium is not much developed, but forms a thick, hard, and refringent cuticula, which is thrown up into spiny ridges. In the second division (mid-gut, “stomach”) the epithelium is composed of very high columnar cells, which make up the greater part of the thickness of the walls, while the cuticula is very delicate, slightly refringent, with no ridges, and is probably not chitinous; the fibres of the muscular coats are not striated, while this division is also distinguished by the presence of glandular follicles and folds. The stomach and the cæcal appendages have all these peculiarities in common, while no other part of the canal is thus characterized.
The third division (intestine and rectum) is composed of an epithelium, the cells of which are intermediate in size between those of the fore and mid gut. The cells are often pigmented, and they are covered by a much thicker cuticula than that of the stomach, but which is not so thick and hard as that of the œsophagus and proventriculus. The very refringent cuticula is not thrown up into ridges, though in some parts it is covered with delicate conical spines, which are very short. “The epithelium and underlying connective tissue (tunica propria) are thrown up into six folds, which run longitudinally, being regular in the ileum and rectum (as the rectal glands), but very irregular in the colon. Outside the depression between each two neighboring folds there is a longitudinal muscular band, these making six bands. This peculiar disposition of the longitudinal muscles does not occur in any other part of the canal; it is, therefore, especially characteristic of the third division.” (Minot.)