548 THEREVIDiE 



4 (3) Species by no means wholly tawny. Prsescutellar bristles in one 



pair (except sometimes in T. arcuatd). 



5 (8) Fifth and subsequent abdominal segments bearing short rigid 



black hairs. Abdomen black, with light grey hindmargins 

 which widen moderately at the sides. 



6 (7) Frontal callus very large (fig. 322) extending up to the front 



ocellus. Wing-veins hardly darkened. 4 plebeia. 



7 (6) Frontal callus small. Wing-veins darkened. 5 cimi.mscri2otLL 



8 (5) Fifth and sixth abdominal segments (normally) bearing short rigid 



pale hairs. Frontal callus moderate in size. 



9 (10) Thorax with two rather obvious grey stripes. Abdomen with the 



black markings on the second and third segments extending to 

 the sidemargins. Prsescutellar bristles sometimes in two pairs. 

 Fourth posterior cell sometimes just open. Tarsi sometimes 

 ferruginous at only the extreme base, or (var. inornata) con- 

 siderably ferruginous at the base. 2 arcuata. 



10 (9) Thorax very indistinctly striped. Abdomen golden with the 



black markings on the second and third segments not extending 

 to the sidemargins. Prsescutellar bristles in one pair only. 

 Fourth posterior cell closed. Tarsi considerably ferruginous 

 at the base. 3 nohilitata. 



11 (2) Frontal callus divided into a pair of spots (fig. 323). Seventh 



abdominal segment pale haired. 6 hipimctata. 



12 (1) Frons brown on the upper part, but without any shining black 



callus. 



13 (14) Halteres blackish. Fourth posterior cell more or less wide open. 



7 lunvlata. 



14 (13) Halteres whitish. Fourth posterior cell closed or almost so. 



8 annulata. 



I have called attention in my notes upon the variation of T. biptmctata to a 

 British specimen which may represent T. margiimla Meig. 



Very few species of Ikereva can be determined with certainty from single 

 specimens unless they are quite normal and in very perfect condition. T. fidva, 

 T. licmdata, and T. annulata should be easily named in both sexes and the 

 females of T. hipimctata and T. plebeia can be distinguished by their frontal callus, 

 while the male of T. bipunctata is not difficult to name because of its rather smaller 

 size and whitish abdominal pubescence ; the female of T. circumscripta is not 

 known to me with certainty, but may be distinguished from that sex of T. plebeia 

 by its smaller frontal callus and probably from T. nobilitata and T. arcimta by 

 sharply defined light grey abdominal bands. There then remain the males of 

 T. arcuata (of which the tawny and black form is very distinct), T. 7iobilitata, 

 T. 2^lebeia, and T. circumscrijita and the females of T. nobilitata and T. arcuata, 

 and as far as my experience goes these are all difficult species, and their determina- 

 tion can only be arrived at by a close examination of the descriptions. 



1. T. fulva Meigen. Almost wholly bright tawny-haired, but usually 

 with a slight dorsal line of black hairs on the abdomen though without 

 fine transverse lines of black hairs across the hindmargins of the segments, 

 and with black hairs on the vertex, frons, and sides of the face in at least 



