THE TRACHEAL SYSTEM. 385 



the larval hind-gut is changed into granular cells, the two imaginal 

 sections of the tube seem to be approximated until they come into 

 contact. 



In the above description, we have mainly followed the accounts of 

 Kowalevsky. According to Van Rees, the reconstruction of the fore- and 

 hind-gats is brought about, not merely by the imaginal discs already mentioned, 

 but a simultaneous regeneration of the larval epithelium takes place, only some 

 of the cells of the larval epithelium undergoing disintegration, while others, on 

 the contrary, undergo repeated division and form a portion of the imaginal 

 oesophagus. 



The salivary glands of the larva (Fig. 189, sp) are entirely destroyed by 

 phagocytes. They are reconstructed from the imaginal discs which, according 

 to Kowalevsky, form a ring at the anterior end of the glandular tube (c/. the 

 statements of Schiemenz, No. 125). 



From the accounts hitherto given, it is difficult to make out what kind of 

 transformation is undergone by the Malpighian vessels. According to Van Rees, 

 a regeneration of the larval cells through division may take place, or these 

 elements may disintegrate. 



The method of transformation of the intestinal canal described above seems 

 very widespread among the holonietabolic Insecta. It has been observed not 

 only in Diptera, but also in Lepidoptera (Kowalevsky, Fkenzel), Coleoptera 

 (Ganin), and Hymenoptera (Ganin). The casting of the mid-gut epithelium 

 was also found by Kowalevsky in Corethra, Culex, and Chironomus. 



[In this connection the more recent works of Rengel (No. XXXVII. ), 

 Mobtjsz (No. XXXIV.), and Kakawaiew (No. XXIV.) should be con- 

 sulted. — Ed.] 



The Tracheal System. 



The fact that the tracheal system undergoes important transforma- 

 tion during metamorphosis is demonstrated by the entirely different 

 form assumed by it in the larva, the pupa, and the imago. We have 

 only to recall that the larva of the Muscidae breathe through a pair 

 of stigmata at the posterior end of the body, the pupa through one 

 occurring in the pro-thorax, while the imago possesses six pairs of 

 stigmata (situated on the meso-thorax, meta-thorax, and four abdominal 

 segments). There is no doubt that in the larva and pupa the other 

 stigmata are closed. The tracheal strands connected with the latter, 

 as well as some other parts of the tracheal system, as pointed out by 

 Wbismann, seem to function as imaginal discs for the regeneration of 

 the tracheal matrix (A 7 ax Rees), and a regeneration of this epithelium 

 can also frequently be seen to proceed from simple division of the 

 cells. The disintegration of the degenerating parts of the tracheal 

 system is accomplished under the influence of phagocytes in the way 

 already described. 



2c 



