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 flv 
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: “Within 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 a 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 tho 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, tho 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 tho 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. 1G3. 
73 
