Parasyrphus species are small to medium size, usually robust, usually with posteriorly rounded yellow maculae or straight or posteriorly emarginate yellow fasciae on terga, rarely with yellow abdominal markings reduced or absent.
Adapted from Vockeroth (1992).
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.
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.
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.
Body length: 6.9-11.0 mm (Vockeroth 1992).
Rotheray and Gilbert (1989) resolved Parasyrphus in a polytomy, but related with Epistrophe, Epistrophella and Meligramma using larval characters. In another analysis, Rotheray and Gilbert (1999) placed Parasyrphus alone but as sister group of a large clade including Fagisyrphus, Syrphus and many other genera.
Mengual et al. (2008), using DNA sequences, resolved Parasyrphus as sister group of the genus Syrphus, in a similar way (but not exactly the same) as Rotheray and Gilbert did (1999).
Flowers visited by adults: white umbellifers; Barbarea, Caltha, Ledum palustre, Papaver nudicaule, Potentilla cranzti, Ranunculus, Rhododendron tomentosum, Rubus chamaemorus, Salix, Saxifraga azoides, Taraxacum (Speight 2010).
Larvae of P. tarsatus have been reported feeding on aphids (Nielsen et al. 1954; Summerhayes and Gilbert 1989).
The flight period for European specimens of P. tarsatus is from the end of May to mid August (Speight 2010).
This species is found in the Alps (France, Switzerland), but its distribution in much of central Europe is uncertain due to confusion with Parasyrphus kirghizorum. It is confirmed from most of Scandinavia; and occurs in the Nearctic from Greenland (Haarto and Koponen, 2003) and Alaska south to mountainous parts of Washington, Colorado and New Hampshire (Vockeroth, 1992).
Adult males hover at 1-2 m. and settle on bare ground, stones or rocks in the sun; both sexes are often found sun-bathing on foliage of Betula in the morning (T. Nielsen, pers.comm.; in Speight 2010).
Adults' preferred environment: forest/open ground; deciduous forest and conifer forest, subalpine Betula/Pinus forest, western taiga and dwarf Betula/Salix scrub tundra (T.Nielsen, 1998 and pers.comm.) and Larix forest (Speight 2010).