1. THEREVA 581 



just the same as in the male, but more conspicuous because of _ the shorter 



fmbescence : prsescutellar bristles normally in two well-defined pairs. Scutel- 

 um with the pubescence greyish white, rather longer and more erect than 

 that on the disc of the thorax. 



Abdomen dull greyish white with a slight silvery sheen except on the shining 

 black eighth segment and ovipositor, but the foremargin of the second segment 

 bears a small obscure brownish spot, and each of the third and fourth segments 

 bears a similar but better defined spot; sometimes the darkened mark- 

 ing on the third segment extends along the foremargin to the sides. Pubescence 

 long and rather erect on the basal segment and on the sides of the second 

 segment, but on the rest of the second segment and on all the third segment 

 and on nearly all the fourth segment short, pale, and recumbent, but 

 black, short, rigid, and erect on the fifth, sixth, and seventh segments, and on 

 the sides of the fourth, while the shining black eighth segment bears still 

 stronger short black rigid hairs, and the ovipositor has its usual circlet of 

 stout conspicuous rather blunt black bristles. Belly dull blackish, but all 

 obscured by whitish dust ; sidemargins conspicuously revolute, i.e., the dorsum 

 encircles the sidemargins and folds a conspicuously raised margin against the 

 belly leaving only about half of the underside visible as belly, and this sunk 

 portion has the hindmargins of the second, third, and fourth segments pale 

 yellow ; pubescence similar to that on the upper side, but the fourth segment 

 and the sides of the third bear the erect rigid black hairs ; seventh segment 

 dropped at its hindmargin ; eighth segment shining brownish black. 



Legs as in the male, but the pubescence on the anterior femora not so long 

 and abundant but still quite enough to be conspicuous ; hind femora with 

 hardly any pubescence ; bristles as in the male, but the front tibias with three 

 or four antero-dorsal bristles on the basal half, three or four postero-dorsal, 

 and three postero-ventral rather low down. 



Wings less glassy whitish, and with the blackish veins less sharply defined 

 when seen from above though especially distinct when seen from the tip. 



Length about 9-5 mm. 



This species is very easily known from all except T. lunulata by the 

 uniformly silvery white pubescence of the male and by the absence of 

 any polished callus on the frons of the female. D. anilis has the face bare, 

 and the fourth posterior cell wide open, while its female has orange 

 femora. T. lunulata Zett. occurs in extreme North Europe, and may be 

 readily distinguished by its blacker halteres, more or less open fourth 

 posterior cell, and by the numerous rather rigid black hairs intermixed 

 with the pale pubescence on the thorax, while the female has the abdomen 

 much more extensively darkened. T. annulata varies a little in the number 

 of prsescutellar bristles, as I have seen specimens of each sex with two 

 such bristles on one side and only one on the other side, and others with 

 only one pair of such bristles. 



T. annulata may be expected to occur wherever there are white-hot sand 

 dunes, and is recorded from Cornwall to Sutherland and from Yarmouth 

 on the East to Barmouth on the West. It naturally is most frequent on 

 the coasts, but I have several inland localities such at Lyndhurst, 

 Barton Mills in Suffolk, Eannoch and other Scotch localities, while Colonel 

 Yerbury found it common on sandhills at Waterville, Co. Kerry, and 

 other Irish locahties. The males execute a wild frantic dance in groups 

 numbering up to eight or ten individuals over hot sand in bright sunshine 

 at from four to six feet from the ground, and when I first saw them near 

 Porthcawl in Glamorgan in June 1906 I was greatly interested in their 

 rapid flight and wide swoops. Mr C. H. Mortimer (Entom. Mon. Mag., 

 1905, 261) records "the beautiful silvery 7s of Oxyhelus mucronatus 

 " dragging the bodies of an almost equally silvery ((^) fly ... to their 



