THEREVID.E 



537 



VIII. THEREVID^. 



Orthorrhaphous brachycerous but by uo means eremochsetous flies of 

 moderate size and more or less conical shape, usually resembling the 

 Bomhylidce in their furry pubescence but distinguished by the five 

 posterior cells; also resembling some Asilidm but distinguished by the 

 eyes being not protuberant from the sunk vertex. 



Head usually almost as broad as the thorax, somewhat semicircular, and rather 

 pufled out behind the eyes, but with hardly any obvious collar and only a short 



Fig. 310. — Thercva nohilitata S- x 5- 



neck ; cephalic bristles practically absent except for the strong black postocular 

 bristles and some bristles which usually occur quite out on the upper part of the 

 actual back of the head. Frons by no means excavated between the eyes, but 

 usually rather (and sometimes strongly) produced near the base of the antennae, 

 ranging from densely hairy (Thereva) to quite bare (Psilocephala) ; _ ocelli three. 

 Face hairy or bare, but never with a distinct mouth-beard. Proboscis rather pro- 

 minent, upturned and rather thick, but not porrect and produced ; sucker-flaps 

 broad ; palpi not jointed ; but in Xestomym the proboscis and palpi are elongate and 

 thin. Eyes bare, not bulging out from the vertex, touching or considerably 

 approximated in the male, but widely separated in the female. Antennas porrect, 

 approximated at the base, and composed of three joints with an apical (sometimes 

 jointed) style ; basal joint large and long ; third joint elongate, and often with an 

 indistinct basal annulation whereby the antennas may appear to be four-jointed. 



Thorax (fig. 311) rather flattened, usually (but not always) densely pubescent in 

 the male but with the pubescence shorter and more adpressed in the female, and 

 with some strong black (usually conspicuous) chastotactic bristles, Avhich show no 



