Fig. 634.—Cross-section through the mid-intestine of pupa of Musca: e, rejected and degenerate epithelium of the larval stomach; f, cellular layer newly formed around the same; m, muscular layer; m′, imaginal cell of m; o, imaginal bud of the mid-intestinal epithelium; t, tracheal stem.—After Kowalevsky, from Korschelt and Heider.
Fig. 635.—Longitudinal section through the proventriculus of a muscid larva: im, fore-intestinal, imaginal ring; oe, œsophagus; pr, proventriculus.—After Kowalevsky, from Korschelt and Heider.
The formation of the mid-intestine (stomach) takes place in such a way that the island-like imaginal buds spread out by cell-multiplication over the outer or basal surface of the larval mid-intestinal epithelium (Fig. 634, o), until they finally unite, so as to form the wall of the imaginal mid-intestine (stomach). At the same time the entire larval epithelium (e) is cast in the interior and forms the so-called yellow body, which becomes surrounded by a layer of small cells and a jelly-like mass, and remains until its destruction in the pupal stomach. The larval muscular layer (m) remains intact as long as the imaginal mid-intestine is not fully developed, when it is attacked and destroyed by phagocytes. The final muscular layer arises from single cells lying on the outer surface of the imaginal buds (Figs. 633, im, 634, m′), which should be regarded as special imaginal cells of the mid-intestinal muscular layer.
The transformation of the fore-intestine is introduced by a degeneration of the proventriculus and sucking stomach. The proventriculus (Fig. 635, pr), which had been formed from a circular fold of the fore-intestine, disappears by the smoothing out of this folded structure. The sucking stomach also similarly degenerates by withdrawing gradually into the œsophagus, so that instead of the original diverticulum there remains only an enlargement of the œsophageal cavity. At the same time this part of the canal is attacked and destroyed by phagocytes, while the destroyed portions become replaced by the gradually extending imaginal parts of the wall. The imaginal ring of the fore-intestine (Fig. 635, im), which, according to Kowalevsky, is concerned in the formation of a great part of the definitive œsophagus, becomes closed at its hinder end so that the communication with the mid-intestine appears to be interrupted.
The hind-intestine of the imago is rebuilt in exactly the same manner. Here also the imaginal ring widens and forms a tube, which while it grows around the openings into the urinary tubes, closes itself against the mid-intestine, while behind it remains in connection with the larval hind-intestine. In a similar way the larval hind-intestine is attacked by the growth from behind of an imaginal ring, which proceeds from imaginal buds near the anus, until finally, when the entire larval hind-intestine is reduced to granule-balls, the two imaginal sections of the tube are brought into contact with each other. (Kowalevsky in Korschelt and Heider.)
The larval salivary glands (Fig. 633, sp) are completely destroyed by phagocytes. Then succeeds the new formation of these glands from imaginal buds, which, according to Kowalevsky, form rings situated at their anterior ends.
The nature of the transformation undergone by the urinary tubes is not yet well ascertained. According to Van Rees, there is in this case perhaps a regeneration of the larval cells by division, but on the other side there may be a histolysis of these elements.
The above-described method of transformation of the digestive canal seems, according to Korschelt and Heider, to be very common among the holometabolic insects. It has not only been observed in the Diptera, but also in the Lepidoptera (Kowalevsky, Frenzel), Coleoptera (Ganin), and Hymenoptera (Ganin). The stripping off of the epithelium of the mid-intestine was found by Kowalevsky to occur also in Corethra, Culex, and Chironomus.