58 DIPTERA OF MINNESOTA. 



An extremely interesting point in the life history of insects in 

 this family, according to Theobald, is their parthenogenetic develop- 

 ment. In the larval stage parthenogenesis is not at all uncommon 

 among insects in general, and is found to some extent in the order of 

 Diptera, but nowhere so peculiarly interesting as in the larval life of 

 the Cecidomyidae. The ovaries in the larvae become fully developed 

 and give ofif eggs ; these eggs hatch into other larvae, which in turn, 

 give rise to further larvae in the same way ; "and so on," says Theo- 

 bald, "until as many as five generations ma}^ be seen in an old larval 

 skin." The discovery of this peculiar phenomenon in the Cecidomyidae, 

 in i860, is credited to a Russian naturalist named Wagner. The larvae 

 of some species leap, being assisted in this act by processes upon the 

 last segment. The pupal case or puparium made by individual mag- 



Fig. 45. Fertilization of Aristolochia clematis by Cecidomyiids. From Theobald's "British Flies." 



gots in this family is brownish. In the case of the Hessian fly it has 

 a slight resemblance to a flax seed, and is commonly thus referred to 

 by wheat raisers. 



Although the Cecidomyids represent a very injurious group of 

 insects, they undoubtedly do good by bringing about cross-fertilization 

 in minute plants, too small for most insects to enter. Theobald de- 

 scribes in the following words the fertilization of the European Aris- 

 tolochia clematis, which is largely accomplished by Cecidomyids. He 

 says that he has seen as many as five of these insects in a single blos- 

 som of this plant : "Each of these greenish blossoms commences with 

 an expanded lip. This passes into a funnel or throat (A)" (see illus- 

 tration), "which is lined by hairs, so placed that they allow the en- 

 trance of an insect, but not its exit. This long funnel opens into a 

 flask-shaped enlargement (B), in which are placed the generative 



