FORMATION OF THE PRO NYMPH FROM THE LARVA. 307 



posterior half of the pronymph, and is ultimately withdrawn. 

 It is found still attached to the cuticle of the larva around the 

 posterior spiracles, on the inner surface of the pupa-case after 

 the escape of the imago. Weismann supposed that the remains 

 of the longitudinal trunks of the larva are withdrawn at the 

 time the imago escapes from the pupa ; but as these exuviae are 

 entirely disconnected with the pupa-sheath and lie outside it, it 

 is evident that they are expelled from the body of the pro- 

 nymph before the pupa-sheath, the shed epidermis of the 

 nymph, is formed. Indeed, I have found them in the pupa- 

 shell freed from the body of the nymph on the third day 

 of the pupa stage. The shedding of the tracheal exuviae 

 appears to be dependent on the contraction of the abdominal 

 paraderm, and to occur at the same time as the expulsion 

 of the cuticular intima of the rectum of the larva from the 

 pronymph. 



f. The Histolysis of the Fat Bodies and other Larval Tissues. 



The Fat Bodies during the formation of the pronymph 

 separate into their component cells, so that the contents of 

 the body cavity has the appearance of a granular fluid even 

 before the histolysis of the muscles is complete. The fat 

 cells, however, undergo histolysis very slowly, so that, as 

 Weismann and Ganin observed, many persist even in the 

 imago when it emerges from the pupa. The histolysis of the 

 fat bodies commences, like that of the muscles, at the anterior 

 extremity of the larva, and proceeds from without inwards, so 

 that in the pronymph their cells are separated from each other, 

 except where they are in immediate relation with the remains 

 of the alimentary canal of the larva. 



Kowalevski claims to have observed the immigration of the 

 leucocytes into the fat cells in the following manner. He says : 

 ' The breaking up of the fat bodies can be observed in the 

 living nymph. I have several times withdrawn one from the 

 pupa-case on the third or fourth day, and kept it alive in 

 white of egg for more than twenty-four hours. The head 

 vesicle is fairly transparent, and it is possible to watch the 



