42 AN ACCOUNT OF BRITISH FLIES. 
gnats.” But in spite of these Cecid characters, Zygoneura agrees 
closely with Sciara, which is a true Mycetophilid. 
Notwithstanding these few exceptions, the Cecidomyide 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 panchenegeaenc develop- 
ment of the larvee. 
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 Cynipfida@, a 
family of small hymenopterous insects, which attack plants in a 
similar way. 
AJl the Cecidomyide 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 larve 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. ¢vitici), 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, ifanything, of more importance, as in many cases it so 
well illustrates that curious phenomenon in the animal kingdom 
known as parthenogenetic development. It was first discovered and 
studied in these larvee 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 larvz, 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 
larvee, which again give rise to another generation, and so on until as 
many as five generations of larvee 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., 1863 ; also Schneider, Zool. Beitrage, i i, 
+ (3), 1885, p. 272; and Mecznikow, Z. n. Z., xvi., 1866, p. 407. 
S Ppeieencsk (z.e., the production of ova by the immature animal, and is in 
the insecta always parthenogenetic). 
