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The Hessian fly, or Cecidomyia destructor, (Say) is another 

 European species, which has become naturalized in the United 

 States, having been introduced seventy-five years ago. It fur- 

 nishes an example of a third kind of transformation, but little, if 

 at all, understood by European entomologists. The larva of this 

 insect, when it has come to its growth, remains fixed and motion- 

 less on the culm of the wheat. Its body contracts, and soon takes 

 the form and color of a flaxseed. While this change is going 

 on externally, the body of the insect gradually cleaves from its 

 outer dry and brownish skin. When this is carefully opened, the 

 included insect will be seen to be still in the larva state. Mr. 

 Westwood found it in this condition in a specimen sent to him 

 from Vienna ; and hence came to the erroneous conclusion that 

 the European species was not the same as the American Hessian 

 fly. But the fact is well established, that the American insect 

 retains the larva form, for some time, within its external or penul- 

 timate flaxseed skin. It does not change its condition, indeed, 

 until a few days before it discloses the winged insect. It does 

 not appear to moult its last larva-skin, in order to become a pupa ; 

 for on careful examination, not the least vestige of a cast skin has 

 been found within the flaxseed shell. The transition from the 

 larva to the pupa state is effected in the same way as in the fore- 

 going examples, by the softening of the anterior segments of the 

 larva, to admit of the development of the limbs and wings of the 

 pupa. The insect, in this stage, may be said to be a coarctate 

 pupa, being inclosed within a brownish leathery skin or pvpa- 

 rium, which, however, as before stated, consists, not of the last, 

 but of the penultimate larva-skin. In due time, the pupa breaks 

 open and crawls entirely out of its puparium, and resting between 

 the leaf and the culm, its delicate skin is rent on the back, and 

 the perfect fly extricates itself therefrom. In this example (and 

 it seems not to be the only one in the genus,) two anomalies 

 occur in the metamorphosis : first, the penultimate larva-skin be- 

 comes the puparium ; and, secondly, the last larva-skin is only 

 modified anteriorly, without being cast off, when the insect is 

 changed to a pupa. It should be noted also, that in all these 

 cases the pupa becomes active shortly before its final change. 



The characteristics of the three varieties of transformations 

 described in this communication may be thus briefly summed up. 



