According to Schneider the longitudinal muscular fibres of the fore and hind gut in insects pass into the stomach (mid-gut). The anterior part of the fore-gut has generally only circular fibres. When, however, the longitudinal fibres arise behind the middle, then they separate from the digestive canal and are inserted a little behind the beginning of the chylific stomach. Hence there is formed an invagination of the proventriculus, which projects into the cavity of the stomach.
Schneider describes this process, which he calls the “beak,” as an invagination of the fore-stomach which projects into the cavity of the stomach. The two layers of the invagination in growing together form a beak varying in shape, being either simple or lobed and armed with bristles or teeth. This beak is tolerably large in Lepisma, Dermaptera (Forficula), Orthoptera, and in the larvæ and adults of Diptera, but smaller in the Neuroptera and Coleoptera, while in other insects it is wanting.
Proventricular valvule.—Gehuchten also describes in Ptychoptera what he calls “the proventricular valvule,” stating that it is “a circular fold of the intestinal wall” (Fig. 310, vpr). He claims that it has not before been found, the “proventricular beak” of Schneider being regarded by him as the œsophageal valvule.
Fig. 316.—Digestive canal of Ptychoptera contaminata: gs, salivary glands; ra, œsophagus; pr, proventriculus; gt, crown of eight small tubular glands; im, mid-intestine; ga, two accessory white glands; vm, urinary vessels; ig, small intestine; gi, large intestine; r, rectum; A, the proventriculus in which the hinder end of the œsophagus extends as far as the chyle-stomach. B, longitudinal section of the proventricular region; sph, muscular ring or œsophageal sphincter; ppr, wall of the proventriculus; e, circular constriction dividing the cavity of the proventriculus in two; vpr, circular fold of the wall of the mid-intestine forming the proventricular valve; vœ, œsophageal valve.—After Gehuchten.
The peritrophic membrane.—This membrane appears first to have been noticed by Ramdohr in 1811 in Hemerobius perla. It has been found by Schneider, who calls it the “funnel.” On the hinder end of the fore-stomach, he says, the cuticula forms a fold enclosing the outlet of the fore-stomach, and extending back like a tube to the anus. This “funnel,” he adds, occurs in a great number of insects. It has been found in Thysanura, but is wanting in Hemiptera. In the Coleoptera it is absent in Carabidæ and Dyticidæ. It is generally present in Diptera and in the larvæ of Lepidoptera, but not in the adults. In Hymenoptera it has been found in ants and wasps, but is absent in Cynipidæ, Ichneumonidæ, and Tenthredinidæ. All those insects (including their larvæ) possessing this funnel eat solid, indigestible food, while those which do not possess it take fluid nourishment. It is elastic, and firmly encloses the contents of the digestive tract. Until Schneider’s discovery of its general occurrence, it had only been known to exist in the viviparous Cecidomyia larvæ (Miastor). Wagner, its discoverer, noticed in the stomach of this insect a second tube which contained food. Pagenstecher was inclined to regard the tube as a secretion of the salivary glands. Metschnikoff, however, more correctly stated that the tube consisted of chitin, but he regarded it as adapted for the removal of the secretions. (Schneider.) Plateau, however, as well as Balbiani, the latter calling it the “peritropic membrane,” considers this membrane as a secretion of the chylific stomach, and that it is formed at the surface of the epithelial cells. It surrounds the food along the entire digestive tract, forming an envelope around the fæcal masses. On the other hand, Gehuchten states that in the larva of Ptychoptera its mode of origin differs from that described by Plateau and by Schneider, and that it is a product of secretion of special cells in the proventriculus.
The mid-intestine.—This section of the digestive canal, often, though erroneously, called the “chylific stomach” or ventriculus, differs not only in its embryonic history, but also in its structure and physiology from the fore and hind intestine of arthropods, and also presents no analogy to the stomach of the vertebrate animals. In insects it is a simple tube, not usually lined with chitin, since it is not formed by the invagination of the ectoderm, as are the fore and hind intestine, the absence of the chitinous intima promoting the absorption of soluble food. Into the anterior end either open two or more large cœcal tubes (Fig. 299), or its whole outer surface is beset with very numerous fine glandular filaments like villi (Fig. 317 and Fig. 329).
The mid-intestine varies much in size and shape; it is very long in the lamellicorn beetles (Melolontha and Geotrupes), and while in Meloë it is very large, occupying the greatest part of the body-cavity, in the longicorn beetles and in Lepidoptera it is very small. The pyloric end consists of an internal circular fold projecting into the cavity. In the Psocidæ (Cæcilius) the pyloric end is prolonged into a slender tube nearly as long as the larger anterior portion.
The limits between the mid and hind intestine are in some insects difficult to define, the urinary tubes sometimes appearing to open into the end of the mid-intestine (“stomach”). The latter also is sometimes lined with an intima. The limits are also determined by a circular projection, directly behind which is an enlargement of the intestine in the shape of a trench (rigole), or circular cul-de-sac (the “pyloric valvule” of some authors, including Beauregard), while the walls of the small intestine contract so as to produce a considerable constriction of the cavity of the canal. This constriction exactly coincides with the beginning of the double layer of circular muscles in the wall of the small intestine. An internal layer, which is the continuation of the circular muscles of the chylific stomach, and an external layer much more developed probably belong to this part of the alimentary canal. Since the homologue of the circular fold occurs in the locust as well as in Diptera, it is probably common to insects in general.
