42 AN ACCOUNT OF BRITISH FLIES. 



gnats." But in spite of these Cecid characters, Zygoneiira agrees 

 closely with Sciara, which is a true Mjcetophilid. 



Notwithstanding these few exceptions, the Cecidomyidm are a well- 

 defined family, and one that has a great number of extremely 

 interesting points, both froni an entomologist's and from an 

 agriculturist's point of view, while the student of biology will also 

 find much of great interest, such as the parthenogenetic develop- 

 ment of the larvae. 



The natural history of these minute insects has been studied in 

 close connection with their classification. This is owing to the fact 

 that one of the most important stages in their life-history is the 

 formation of "galls," in this respect resembling the Cy?iipidce, a 

 family of small hymenopterous insects, which attack plants in a 

 similar way. 



All the Cecidomyidce are small insects, and not easily identified. 

 Their wings are covered by minute hairs and scales, which very easily 

 rub off. They are most easily identified by studying the " galls " in 

 which the larvce live, and for this reason we are so well acquainted 

 with many of the life-histories of the family. There are certain 

 species which do not produce gall formations, but live upon cereals. 

 Of these we must particularly note the Hessian-fly ( C. destructor) and 

 the wheat-fly (C tritici), which are so destructive to crops. 



Perhaps no family of Diptera are so interesting as the one we are 

 considering ; not only are they interesting as perfect insects, but their 

 larval stage is, if anything, of more importance, as in many cases it so 

 well illustrates that curious phenomenon in the animal kingdom 

 \.no\\n zs partticnogenetic Aeve\o'gme.n\.. It was first discovered and 

 studied in these larvK by Wagner,* and has continually called forth a 

 great deal of attention at the hands of embryologists. It takes place 

 as follows : The female places her eggs under the bark of a tree, where 

 they first develop into ordinary larva?, but internally curious changes 

 take place. The ovaries become fully developed and bud off eggs ;t 

 these eggs live upon the visceral mass, and soon hatch into secondary 

 larvae, which again give rise to another generation, and so on until as 

 many as five generations of larvae may be seen in the old larval skin. 

 In the following summer a metamorphosis takes place and the sexual 

 generation appears. There are other interesting cases of partheno- 

 genesis in the Diptera, but none so interesting as those of the 

 Cecidomyia. 



* Zeitschr. f. Wiss. Zool., T. xiii., 1S63 ; also Schneider, Zool. Beitrage, i. 

 (3), 1S85, p. 272 ; and Mecznikow, Z. n. Z., xvi., 1866, p. 407. 



t = Patdogenesis (i.e., the production of ova by the immature animal, and is in 

 the insecta always parthenogenetic). 



