Parasyrphus tarsatus (Zetterstedt)  

Adapted from Vockeroth (1992).

MALE.

Head: Frons less swollen, with shorter and less dense pile, black in most arctic specimens, grayish in more southern specimens; eye with long dense pile; eye angle about 100°; eyes contiguous dorsally or, in some arctic specimens, separated by about width of two ommatidia. Face less strongly protruding below, dull yellow with broad black ventral margin, darkened laterally in some specimens, with long dense black pile. Antenna black.

Thorax: Scutum subshining; scutellum dull yellow to dark brown with blackish posterior margin; pleura subshining; thoracic pile at least partly yellow, in southern specimens mostly or entirely yellow. Ventral scutellar fringe complete. Pleura black, weakly pruinose. Anterior anepisternum with many long fine erect hairs. Dorsal and ventral katepisternal pile patches separated or narrowly joined posteriorly. Meron, katatergum, and metasternum bare. Wing membrane entirely microtrichose. Legs usually black, with femora yellow on up to apical half and pro- and mesotibiae entirely yellow.

Abdomen oval, unmargined. Terga 2-4 usually with large yellow semicircular maculae; in northern specimens maculae on tergum 2 commonly not reaching lateral margins; maculae on terga 3 and 4 smaller; in north Greenland specimens tergum 2 with maculae tiny and obscure or absent but tergum 3 always with at least small obscure maculae.

FEMALE.

Not always distinguishable from female of P. groenlandicus at or above northern limit of trees. Face usually only little more than half as wide as head but slightly variable; color of face, thoracic pile, and legs varying as in male. Abdomen of specimens from below treeline with large distinct yellow maculae, similar to those of male but with straighter posterior margins; specimens from near treeline or southern arctic tundra commonly with maculae greatly reduced; some specimens from southern Greenland and all from northern Greenland with pale abdominal maculae absent.

Variation.

The marked variation in this species has been studied by F.C. Thompson. He distinguished four morphs, based on color and on characters of the male head and metaleg. He designated these as high arctic, low arctic, Rocky Mountain, and typic and has found intermediates between all but the first two (which may be because too few specimens have been collected). He concluded, and I agree, that specimens of these forms are conspecific. Males of P. tarsatus can be readily distinguished from those of the arctic species P. groenlandicus by the differences in the frons indicated in the key and descriptions (P. groenlandicus has frons large, swollen, very broad, with long dense pile; eye angle about 130°). Thompson concluded that females of the two species could be distinguished by the wider face of P. groenlandicus relative to head width, but, in my opinion, the slight variation in this character and the difficulty of measuring it make it unreliable. Females from south of the area of distribution of P. groenlandicus have moderately large yellow or orange maculae on the terga. Females from localities where males of both species occur have the maculae either very variable in size or absent; they cannot be definitely referred to one species or the other. Females from high arctic localities in Canada, where males of only P. groenlandicus have been taken, either have very small obscure maculae or, more commonly, lack maculae. Finally; the females of a long series of specimens from southern Greenland, of which the males have the characters of P. tarsatus, have the abdomen either with small orange maculae or entirely black. Females of an even longer series of 172 specimens, from northern Greenland, with the males referrable to P. tarsatus, have the abdominal maculae absent.