346 THE DEVELOPMENT OE THE NYMPH. 



The stomodseum and proctodaeum are also far more ad- 

 vanced than the mesenteron ; this on the tenth or eleventh 

 day is a mere thin-walled sac, with a coiled caecal prolongation 

 of its posterior extremity, which becomes the metenteron. At 

 the junction of the latter with the mesenteron the Malpighian 

 vessels are seen as short caeca ; they are chiefl}^ developed 

 during the last two days of the pupa stage, but the rectal 

 papillae are almost as well developed on the tenth day as in the 

 perfect insect. 



The thoracic muscles and ganglia are very imperfect at this 

 period, and Weismann observed that if the nymph is removed 

 from the pupa-case even on the eleventh day, it exhibits no 

 trace of muscular movement. The great size of the blood 

 sinuses and the late redevelopment of the dorsal vessel have 

 been already referred to. 



The tracheal system of the imago remains very inconspicuous 

 until after its escape from the pupa, so that Weismann states 

 that, ' of all the organs, the tracheae are the last developed ' 

 [2, p. 235]. In this he was wrong, as the principal tracheal 

 trunks of the fly are all present on the ninth or tenth day, 

 although the smaller vessels are apparently developed later. 



The sexual glands form a marked exception to all the other 

 internal organs, as they exist in a rudimentary condition 

 in the young larva, and probably in the embryo, and undergo 

 progressive development, which is complete in the male a few 

 hours after its escape from the pupa, and in the female, onlv 

 some weeks later. 



