DIPTERA. 461 



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 sprino-, 

 the insects are transformed to flies. They make their escape 

 from their winter quarters by breaking through one end of 

 their shells and the remains of the leaves around them. In 

 the " Observations on the Hessian Fly," written by Jonathan 

 N. Havens, Esq., it is stated, that "whenever the fly has been 

 hatched in the house, it always comes forth from its brown 

 case wrapt in a thin white skin, which it soon breaks, and is 

 then at liberty;" and Mr. Havens supposes, that the same 

 thing occurs when the transformation takes place abroad. Mr. 

 Herrick states, that this skin or " scarf," as he calls it, " splits 

 on the thorax or back," and the fly is disengaged from it by 

 working through the rent. This process, and the appearance 

 of the insect through the pupa-skin, is fully described in his 

 letter of the 21st of February, 1843, from which the following 

 extract is taken. It is from a memorandum made May 12, 

 1837. " On looking over culms of wheat, which ripened last 

 July, I found a puparium of the Hessian fly; began to cut it 

 open ; found within a fly nearly matured. Opened only the 

 anterior part of the puparium; but the animal soon squirmed 

 itself out, enveloped in a thin scarf The puparium was left 

 entirely clean. — The animal worked its abdomen back and 

 forth, and, in about twenty minutes, was detached from the 

 scarf" In one instance, Mr. Herrick found the empty scarf- 

 skin "attached to one end of the puparium." Ordinarily, 

 however, the insect seems to crawl entirely out of the pupa- 

 rium or flax-seed shell, before disengaging itself from the pupa 

 skin, as stated above by Mr. Havens. Upon examining a 

 puparium after the escape of the insect, I could not discover 

 any vestige of larva or pupa skin within it. It was left en- 

 tirely empty. 



