20 6 NORTH AMERICAN DIPTERA. 



The family comprises about two hundred known spe- 

 cies, with but few widely distributed genera. The flies 

 resemble the Asilidae somewhat, and have habits not 

 dissimilar, though much less active. The proboscis has 

 fleshy labella, instead of the horny tip of the Asilidae, 

 and the legs are less stout — in many species they are 

 easily broken off when captured. Their food is chiefly 

 other diptera, for which they lie in wait upon leaves and 

 bushes, or upon the bare ground. The larvae have a 

 short, eyeless, nonretractile head, the antennae small and 

 short. The body is slender and snake-like, showing ap- 

 parently nineteen segments. Anterior spiracles situated 

 at the end of the first segment behind the head; poste- 

 rior spiracles on the apparently seventeenth segment. 

 The larvae live in the earth and decomposing wood, or 

 in sand, feeding upon other insects or upon vegetable 

 matter, ordure, etc. The pupae are free; they have in 

 front laterally projecting spinous points. 



The genera of this family are, for the most part, very 

 unsatisfactorily founded. Few structural differences ex- 

 ist, save in the antennae and proboscis, and these differ- 

 ences seem usually to have specific value only. Five 

 genera have been proposed for North American species; 

 each contains a single species, and in all probability 

 there never will be any additions to them. On the other 

 hand Thereva and Psilocephala, with numerous species, are 

 doubtfully distinguished by the vestiture of the face! 

 If the smaller genera are recognized, at least one or 

 two more should be formed for those species of Thereva 

 having a thickened first autennal joint ( T. melanophcba 

 Loew, T. crassicornis Bell., T. pachyccras, n.n. for T. 

 crassicornis Will. ) The closure or non-closure of the 

 fourth posterior cell occurs in both genera and compli- 

 cates matters. The division of the first posterior cell in 



