VII EMPIDAE DOLICHOPIDAE 493 



large hind legs. In Hemerodromia the front legs are raptorial, 

 the femora being armed with spines on which the tibiae 

 close so as to form a sort of trap. Many Empidae execute 

 aerial dances, and some of the species of the genus Hilara are 

 notorious for carrying veils or nets in the form of silken webs 

 more or less densely woven. This subject is comparatively new, 

 the fact having been discovered by Baron Osten Sacken in 

 1877,^ and it is not at all clear what purpose these peculiar 

 constructions serve ; it appears probable that they are carried 

 by means of the hind legs, and only by the males. Mik thinks 

 that in H. sartor the veil acts as a sort of parachute, and is of 

 use in carrying on the aerial performance, or enhancing its 

 effect ; while in the case of other species, H. maura and H. inter- 

 stirtcta, the oliject appears to be the capture or retention of prey, 

 after the manner of spiders. The source of the silk is not 

 known, and in fact all the details are insufficiently ascertained. 

 The larvae of Empidae are described as cylindrical maggots, with 

 very small head, and imperfect ventral feet ; the stigmata are 

 amphipneustic, the thoracic pair being, however, excessively 

 small ; beneath the posterior pair there is nearly always a tooth- 

 or spine-like prominence present. 



Fam. 27. Dolichopidae. — Graceful flics of metaUic colours, 

 of moderate or small size, and long legs ; usucdly ■with bristles on the 

 thorax and legs, the halter es exposed, squamae 

 leing quite absent ; antennae of two short stout 

 joints (cf tahich the second is rccdly tivo, its 

 division being more or less distinct'), ivith a 

 thread-like or hair-like ci'ppendage. Proboscis 

 short, fleshy. Claivs, pulvilli, and empodium 

 small ; toings vjith a simple system of nervures, 

 those on the posterior part of the wing are but 

 few, there is no anterior basal cross-vein be- 

 tween the disced and second basal cells, ivhich 

 therefore form but one cell. This is also a *^ig.2:35.-A Larva, 



J <! B, pupa of Mede- 



very extensive family of flies, of which we terus ambigwis. 

 have probably about 200 species in Britain. penisT' ^^ 



They are conspicuous on account of their 

 golden, or golden-green colours, only a few being yellow or black. 



^ Ent. ilag. xiv. 1877, p. 226 ; for a discussion of the subject see I\Iik, IVien. 

 ent. Zeit. xiii. 1894, p. 273. 



