194 



of double moulting on assuming the winged state, for the insect 

 sheds both its larva- and pupa-skins at the moment it appears 

 as a fly. 



Still more remarkable is the moulting of Uphcmeridce. When 

 about to undergo its transformations, the pupa of an Ephemera 

 crawls out of the water, and, after resting a while on some stick 

 or fence, it throws off its pupa-skin and flies away apparently 

 fully formed. Soon, however, it settles again, and then casts off 

 another skin, exceedingly thin and delicate, which not only cov- 

 ered its whole body, but invested also its legs and filmy wings. 

 Beetles, wasps, bees, and most hymenopterous insects, cast off" 

 only one skin, the true pupa-skin, when they take the winged 

 form. The same is true of gnats, musquitoes, horse-flies, etc., 

 which cast off only the single pupa-skin to become a per- 

 fected insect, having some time before moulted the larva-skin 

 to become pupae. 



Is the Hessian Fly to be cited as an example of transforma- 

 tion like that of the Ephemera, or is it intermediate between 

 that of the Ephemera and that of common flies ? or, on the 

 other hand, is it altogether similar to that of the latter, as de- 

 scribed in my first example ? In other words, does this insect 

 come out of its flaxseed puparium in the form of an active 

 pupa, and then cast its pupa-skin, in order to become a winged 

 gnat ? Or does it come out of the puparium in the form of a 

 gnat, and then throw off a skin, like an Ephemera, before it 

 can be said to have attained the perfect state ? Or, finally, does 

 it burst and extricate itself, at the same moment, from puparium 

 and pupa-skin, leaving the latter within its puparium ? 



From a remark in one of your letters I infer that the latter 

 kind of moulting does not occur to the Hessian Fly ; but you 

 have not stated what the precise change is, nor how we are to 

 understand the statement of Mr. Havens. 



