W. WESCHE ON MODIFICATIONS OF LEGS TN DIPTEROUS FLIES. 247 



the males of which have a very largely developed hypopygium. 

 The whole organ is turned under the end of the abdomen, and 

 its extremity consists of about twenty kuife-like hooks {Dolichoinis 

 aeneus and Poecilohothrus nohilitatus are good examples). So re- 

 markable and highly specialised is this organ that Cuvier figured 

 it in the Journal de Physique. As an example of the economy 

 of nature, it may be noted that the males of those genera 

 possessing this organ have simple legs not differing from those 

 of the females. In Campsicnemus the hypopygium is without 

 the hooked process, but the middle legs are elaborately modified 

 in compensation. The flies belonging to this family are pre- 

 daceous, feeding on minute worms and gasteropods, and have 

 developed the faculty of gliding on water to enable them. to get 

 at their prey. Some species are found in our gardens, where 

 they probably do good by killing the newly-hatched snails. 



Platycliirus is another genus of the Hover flies, and has in the 

 males the tarsi and tibiae of the forelegs much flattened and 

 covered with pads of soft hair (Fig. 5), The female is by this 

 contrivance more firmly held. 



It needs a trained eye to distinguish Hydrotea dentipes from 

 the house-flies. All the males of this genus (family Anthomyidae) 

 have a tooth or teeth on the fore femora and a modification of 

 the hairs and setae (Fig. 6). 



Sei^sis cynijysea is a very small and graceful fly, often found in 

 our gardens. It is the commonest species of the family, and, in 

 common with three others, has a small dark spot on the tip of 

 the wing. The males have a curious tubercle, and teeth on the 

 femora and tibiae of the foreleg (Fig. 7). Both sexes are 

 furnished with brushes of hair on the fore tarsi to keep the eyes 

 and antennae clean. In Borhorus nitidus we find a fresh 

 departure ; the chitin of the femur has been drawn out into 

 the curious hook illustrated in Fig 8, This is on the hind leg. 

 All the setae and spines previously figured grow out of sockets, 

 and are obviously modified hairs, but "this is part of the femur. 

 The female has also a "secondary sexual character" which 

 is comparatively rare. " The small second spur of the hind tibiae 

 is twisted and angular, and catches on the hook of the male " 

 (Walker). 



Limosina sylvatica is also of the same family [Borhoridae). 

 In this species the middle legs are highly modified. The 



