186 DIPTERA OF NORTH AMERICA. 



distance from each other (Cec. salicina, 0. veronica, etc., see Wz., 

 1. c. f. 3, 4). Behind these horns, two pairs of bristle-like pro- 

 cesses may be observed in most pupae. The first pair is also on 

 the head, close by the horns, the second on the thorax. Both 

 vary in size and strength in different species. The second, tho- 

 racic pair, has been taken by some authors for a spiracle. These 

 projections and horns, especially the frontal ones, aid the pupa in 

 working its way through the gall or from underground, before 

 entering its last stage of existence. The dorsal segments of the 

 abdomen are, for the same purpose, frequently rough with spines. 

 The tip of the abdomen is sometimes smooth; in other instances 

 it bears a few bristles. 



After the exclusion of the perfect insect, the pupa-skin remains 

 frequently hanging on the outside of the gall. 



Some species of Cecidomyia have only one, others more than 

 one yearly generation. The summer generation of the latter kind 

 remain but a short time in the pupa state; the winter generation 

 much longer. 



The Iarva3 of Lasioptera resemble those of Cecidomyia in their 

 structure as well as in their habits. They frequently have the 

 same reddish color and the peculiar breast-bone. L. rubi Ileeger 

 and the N. American L. vitis O. S. produce swellings in the stem 

 of the plants which they inhabit. L. pusilla Heeger forms galls 

 on the leaves of Sonchus, L. cerris Kollar on those of the oak 

 ( Quercus cerris). 



III. On the North American CECIDOMYIA hitherto observed and 



their galls. 



The species of N. A. Cecidomyice at present known, may be 

 distributed into three categories, according to the extent of our 

 knowledge concerning them. About the species of the first cate- 

 gory, nothing but the description of the perfect insect is extant, 

 its habits remaining unknown ; as to those of the second category 

 we are acquainted with the first stages of their existence, especially 

 with the deformations they produce, without knowing the perfect 

 insect; finally, to the third category belong those, the habits of 

 which, as well as the perfect insect, are described. 



The following is a synopsis of the species recorded by previous 

 authors, as well as of those mentioned in this paper: 



