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skins once, after taking the imago form. Moreover, many larvae 

 cast their skins repeatedly before assuming the pupa state. Some, 

 however, do not moult the larva-skin till they are about to become 

 pupae. Other insects, during the pupa state, retain the loosened 

 larva-skin, and this forms a case or kind of cocoon for the in- 

 cluded pupa. Dipterous insects are said either to cast off the 

 larva-skin once only, that is, when they disclose the pupa, or, to 

 retain it for a case to the pupa, which hence is called a coarctate 

 pupa. 



The writer does not recollect that any exceptions to the fore- 

 going statements have hitherto been recorded by European ento- 

 mologists. The transformations of the CecidomyicB, as observed 

 by American naturalists, offer certain peculiarities or remarkable 

 exceptions, which are now to be described. There are three 

 modifications in the transformations that these insects undergo, 

 represented respectively by Cecidomyia solids, (Fitch), G, tritici, 

 (Kirby), and O. destructor, (Say). 



Cecidomyia salicis is an American species, inhabiting willow 

 galls. Being of large size, it is an excellent object for the obser- 

 vation of the anomalous transformation that is common to it and 

 to other species of the genus. The gall, produced by this insect, 

 consists of a woody tumor, surmounted by the dry and brittle ter- 

 minal bud, at the tips of the twigs of Salix rigida and Salix lu- 

 cida. It contains only a single larva, which perforates the gall 

 from the tip to the bottom, and, when fully grown, lines a portion 

 of the cavity with a delicate silken web. During the winter, the 

 larva remains unchanged, and in a dormant state, within its cell. 

 The change to the pupa state occurs in the following spring, and 

 is effected without any moulting of the larva-skin. Dr. Asa Fitch, 

 the describer of the species, first called my attention to this fact, 

 which has been confirmed also by my own observations. The 

 approaching change is marked by an alteration of the color of the 

 anterior segments of the larva, which, from orange, become red 

 and shining, as if distended with blood. Soon afterward, rudi- 

 mentary legs, wings, and antennae begin, as it were, to bud and 

 put forth, and rapidly grow to their full pupal dimensions ; and 

 thus the transformation to the pupa is completed. T7hen the fly 

 is about to be disclosed, the pupa bursts through the silken lining 

 of the upper part of its cell, and works its way to the external 



