Species very similar to Platycheirus perpallidus Verral, 1901. In the male, this species is almost indistinguishable from P. immarginatus. Platycheirus perpallidus can exhibit a row of five or six, long bristly (black, or yellowish, or some of each) pili, postero-dorsally, on the profemur, as in P. immarginatus. But, in P. perpallidus these bristly pili each have a diameter distinctly less than the basal width of the clump of tangled white hairs that occurs at the base of the femur, posterodorsally. In P. immarginatus each of the long, black, bristly pili present on the postero-dorsal edge of the profemur has a diameter even greater than the basal width of that clump of tangled white pili. The female of P. perpallidus is most easily distinguished from that of P. immarginatus in having an almost continuously pale lateral margin to the abdominal terga (this may be darkened by post-mortem discolouration), whereas in P. immarginatus the lateral abdominal margin is almost continuously (and widely) black (Speight 2010).
Adapted from Vockeroth (1992).
Head: Face vertical with small shining tubercle, otherwise moderately gray to grayish brown pollinose. Antenna black.
Thorax: Scutum shining, slightly pollinose laterally, usually with only pale pile but with some black pili in some specimens. Scutellum shining, with pale pili. Pleura with yellowish brown pollinosity above and grayish pollinosity below, with pile usually entirely pale but partly brown to black in some specimens on upper half and stiff and black on lower part ofkatepisternum; upper part of katepisternum without pile or with up to eight pili; pile of posterior part of anepisternum moderately dense and crinkly, with vertical submarginal row commonly stronger and almost bristle-like. Halter capitulum yellow. Wing membrane entirely microtrichose. Legs usually mostly yellow with coxae and trochanters black; first, fourth, and fIfth tarsomeres of metaleg brown to dark brown above; pro- and mesofemora in some specimens each with brown posterior vitta and metafemur and tibia each with brown ring on middle half; rarely metafemur and tibia dark brown except for narrow bases and apices. Protrochanter with many short stiff yellow setae on ventral surface; profemur posteriorly with subbasal tuft of about three very closely appressed long wavy white pili followed by nearly regular row of four or five long slender bristles with wavy tips, of which at least last three are black; longest bristle about twice as long as femoral diameter; tibia almost uniformly broadened from base almost to apex, with posteroapical angle subtriangular and bluntly rounded; first protarsomere with posterior margin oblique and anterior margin straight, about halfas wide as long and slightly narrower than tibia; second tarsomere subrectangular, slightly wider than long and slightly narrower than first. Mesofemur anteroventrally on apical two-thirds with nearly regular single or in some specimens partly double row of 10-22 short stiff acute black setae and usually with one or two long slender strongly curved black hairs just before or just beyond end of row, ventrally on basal half with row ofthree to six variable but usually strong yellow or rarely black bristle-like pili; longest pile about twice as long as femoral diameter; mesofemur posteriorly and posteroventrally on apical half with few to many scattered short stiff yellow or black setae; mesotibia slightly broadened on apical three-fifths, anteroventrally usually with short but distinct crinkly yellow pili on most of its length, posteroventrally usually with some similar yellow or dark pili near mid length. First tarsomere of metaleg rather strongly swollen, about 3.3 times as long as its greatest depth.
Abdomen mostly yellow with black areas. Tergum 2 with narrow median vitta strongly broadened anteriorly, and with very narrow posterior margin, brownish black to black; terga 3 and 4 similar to tergum 2 but with median dark vitta narrower and narrowed anteriorly or in some specimens indistinct; tergum 5 yellow with small posteromedian black triangle not reaching anterior margin in some specimens; rarely in northern specimens yellow maculae reduced; maculae of tergum 2 either not reaching to anterior or lateral margins, or small and rounded, or even absent; maculae of terga 3 and 4 not reaching anterior and lateral margins and separated by black median vitta about one-third as wide as tergum; maculae of tergum 5 broadly separated or small and obscure.
Not distinguishable from females of several similar species.
Body length: 6.5-9.6 mm (Vockeroth 1992).
Scirpus maritimus is the only known plant whose flowers are visited by P. immarginatus (Speight 2010).
The flight period for the European specimens is from the end May to the beginning September (Speight 2010).
Holarctic species ranging from Saskatchewan to Quebec, south to Utah and New Jersey. In Europe, its range is uncertain,due to confusion with other species, until recently; in Britain present from the Scottish Highlands to the south coast of England, where it is not infrequent in coastal localities; from various coastal sites in Ireland; its presence cannot yet be confirmed for France or central Europe, but it occurs in Sweden, Denmark, the Netherlands and Belgium (Speight 210).
Adult specimens flies among tall waterside vegetation; as easily detected by use of a sweep net as by direct observation (Speight 2010).
Preferred environment: wetland; in atlantic parts of Europe particularly in freshwater coastal marshes; fen and flush systems in blanket bog; cut-over raised bog; taiga wetlands (Speight 210).