THE DIGESTIVE CANAL 683 



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 degen- 

 eration of the proventriculus and sucking stomach. The proven- 

 triculus (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 with- 

 drawing gradually into the oesophagus, so that instead of the original 

 diverticulum there remains only an enlargement of the oesophageal 

 cavity. At the same time this part of the canal is attacked and 

 destroyed by phagocytes, while the destroyed portions become re- 

 placed 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 oesophagus, 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. 63.3, 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 regen- 

 eration 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 holometa- 

 bolic 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. 



The tracheal system. As we have seen (p. 448), the tracheal sys- 

 tem of caterpillars just before pupation undergoes disintegration, 



