DIPTERA 



CHAP. 



Fam. 17. Therevidae. Moderate-sized flies, with somewhat 



the, appearance of short Asilidae. They hare, //O//Y/VT, on/// a 



feeble fleshy proboscis, mid minute claws, with pulrilli but no 



empodium ; the antennae project, are short, 

 three -jointed, pointed. The flies of this 

 family are believed to be predaceous like 

 the Bobber-flies, but they appear to be very 

 feebly organised for such a life. We have 

 about ten species in Britain, and there 

 are only some 200 known from all the 

 world. But little is known as to the meta- 

 morphoses. Meigen found larvae of T. 



nobilitata in rotten stumps, but other larvae 

 FIG. 231. Thereva (Psilo- 



cephaia) confinis. A. have been recorded as devouring dead pupae 

 Pupa ; B, larva Europe. or i arvae o f Lepidoptera. The larvae are said 



(Alter Perns. ) 



to be elongate, very slender, worm-like, and 



to have nineteen body-segments, the posterior pair of spiracles 

 being placed on what looks like the seventeenth segment, but is 

 really the eighth of the abdomen. The pupa is not enclosed 

 in the larval skin ; that of Psilocephala is armed with setae and 

 spinous processes, and was found in rotten wood by Frauenfeld. 



Fam. 18. Scenopinidae. Rather small flies, without bristle*. 

 Antennae three-jointed, the third joint rather long, without ap- 

 pendage. Proboscis not projecting. Empodium absent. These 

 unattractive flies form one of the smallest families, and are 

 chiefly found on windows. S. fenestralis looks like a tiny 

 Stratiomyid, with a peculiar, dull, metallic surface. The larva 

 of this species has been recorded as feeding on a variety of 

 strange substances, but Osten Sacken is of opinion l that it is 

 really predaceous, and frequents these substances in order to find 

 the larvae that are developing in them. If so, Scenopinus is useful 

 in a small way by destroying " moth," etc, The larva is a little 

 slender, cylindrical, hard, pale worm of nineteen segments, with a 

 small brown head placed like a hook at one extremity of the body 

 and with two short, divergent processes at the other extremity, 

 almost exactly like the larva of Thereva. Full references to the 

 literature about this Insect are given by Osten Sacken. 



Fam. 19. Nemestrinidae. These Insects appear to be allied 

 to the Bombyliidae. They are of medium size, often pilose, and 



1 Ent. Mag. xxiii. 1886, i>. 51. 



