1. THEREVA 577 



Extremely like T. annulata, but apparently a more Arctic 

 species. 



$ . Resembling T. anmdata but tlie face altogether smaller and the eyes curving 

 round more on their lower margin; pubescence on the side-cheeks less 

 abundant ; postocular festoon more conspicuous because composed of stronger 

 bristles ; black bristles quite out on the back of the head fairly numerous. 

 Eye- facets hardly contrasted in size. Antenna with more obvious black 

 bristles near the base of the third joint. 



Thorax with very different pubescence, as when viewed from the side the 

 most conspicuous hairs are all black except on the hind part and on the sides 

 after the suture, the brilliant white pubescence is not quite so dense or 

 woolly, and the black chsetotactic bristles stand out conspicuously; there 

 are two well defined pairs of prsescutellar bristles. 



Abdomen with slightly shorter and less abundant white pubescence. 

 Genitalia very distinct, the large greyish white basal piece being produced 

 into a long triangular lobe on each side, leaving a somewhat quadrate dorsal 

 space between in which some upper lamell£e lie, whilst the lower margin of 

 each lobe slopes back to the base ; beneath these come a pair of large long 

 oval lamella, rather pointed at the end and bearing there some long 

 black bristly hairs; between these lamellae some elongate thin brownish 

 orange processes can be seen (when viewed from behind) which are difficult 

 to determine in the single specimen I have seen. 



Legs more extensively but less sharply blackish at the tip of the tibi^, 

 and the tarsi less yellowish at the base. Anterior femora with one rather 

 small black antero-ventral bristle; hind tibiae with rather more numerous 

 dorsal and postero-dorsal bristles. 



_ Wings without any orange tint, the veins being stronger and blacker : 

 stigma browner ; fourth posterior cell wide open though slightly contracted 

 in all the British specimens I have seen, but only moderately contracted in 

 one specimen and almost closed in another specimen in Zetterstedt's collec- 

 tion atLund. Squamse not so blackish at the base but with a dark marginal 

 speck just before the fold. Halteres greyish black, but lighter grey on the 

 outside and greyish yellow on the underside ; middle of the stem orange. 



?. Very much like T. annulata, but with a more bluish grey tinge. Frons 

 distinctly narrower at the vertex where it is only about one-seventh the width 

 of the head, with the brown upper jiart darker brown and bearing stronger 

 black bristles ; a slight interval occurs before the bristly hairs on the white 

 part of the frons, and these bristly hairs are more or less black with some 

 thin silvery white hairs intermixed, or vice versd, and a large portion of 

 the middle of the white part is bare; the side-cheeks bear less abundant 

 pubescence, and the sides of the mouth are narrower and bear less pubescence ; 

 black bristles quite out on the back of the head considerably stronger and 

 more numerous. _ Antennae with more numerous bristles on the basal joint. 



Thorax greyish black, with two conspicuous pale yellowish grey narrow 

 stripes which start from the front but do not extend quite back to the hind- 

 margm ; _ pubescence on the disc mainly composed of rather thin suberect 

 black hairs, intermixed with which are a few shorter depressed inconspicuous 

 yellow hairs, and these latter become more numerous and longer towards the 

 sides and hind part ; the pubescence altogether is very different from the 

 conspicuous short depressed yellowish pubescence of T. annulata, and does 

 not at all conceal the ground colour ; pleurae with pubescence similar to but 

 whiter than in T. amiulata. Scutellum with considerable rather coarse pale 

 pubescence about the margin, but with suberect mainly black hairs on the disc. 



Abdomen less conical, with the same silvery sheen on the ground colour 

 but the blackish brown spaces large and forming wide arcs ; second 

 segment with a large blackish brown arc which commences close to the basal 

 corners and arches down on the middle until it occupies quite two-thirds of 

 the segment; third segment with a similar but slightly flatter dark arc, 

 which begins still nearer the basal corners and (as the segment is shorter) 

 extends at the middle down three-fourths of the segment ; fourth segment 



2o 



