TRANSFORMATIONS OF THE HESSIAN FLY. 577 



process of growth goes on, and, by and by, on opening the 

 leathery maggot-skin, now a puparium, you find the pupa 

 so far advanced that some of the members of the future fly 

 are discernible through the scarf which envelopes and fetters 

 it on all sides." In his observations communicated to the 

 Commissioner of Patents in 1844,* he referred to the same 

 process in the following words : " AVithin this shell (the 

 flax-seed case) the pupa gradually advances towards the 

 winged state ; it contracts in length, but not in breadth ; and 

 its skin appears covered with minute elevations. Just be- 

 fore evolution (of the fly), we find the pupa invested in a 

 delicate membrane or scarf, which not long previous was 

 its outer skin, through which many parts of the future fly 

 may be distinctly seen." 



From the foregoing passages, it appears that the transi- 

 tion of the insect, within the flax-seed case, from the form 

 of a larva or maggot to that of mature pupa, takes place 

 only a short time before its final transformation to a fly, 

 that is, towards the end of April or beginning of May ; and 

 that the scarf or proper skin of this pupa is the same as 

 that wherein the body of the insect had been previously 

 enveloped. In this respect, the Hessian fly agrees in its 

 transformations with the willow gall-fly ; and doubtless the 

 transition in question is effected in the same way as in that 

 insect. But the larva of the Hessian fly does not spin a 

 silken web or cocoon like that of the willow gall-fly and 

 some other Cecidomyians ; and it differs from these insects 

 also in being finally invested with two skins, the outer 

 one, when detached, serving instead of a cocoon for the 

 included insect ; while the inner one, of a much thinner 

 and more delicate texture, becomes the true skin of the 

 matured pupa. 



Towards the end of April and in the fore part of May, 

 or as soon as the weather becomes warm enough in the 

 spring, the insects are transformed to flies. They make 



* Report, p. 163. 

 73 



