Using an extended proboscis, this slim hoverfly laps pollen from the anthers of a marsh marigold (Caltha palustris) blossom. Adult Platycheirus sp. (Family Syrphidae) feed on nectar and pollen – unlike the predatory larva which feed mostly on aphids. The labellum (seen opening and closing in the photo below) laps pollen off the anthers. When resting the labellum remains closed, and the proboscis folds up under the head.
Addendum: Now identified as a female Platycheirus obscurus (Say, 1824). Thanks to Andrew Young. See comment below.
More information on Syrphid feeding can be found in Foraging Ecology of Hoverflies: morphology of the mouthparts in relation to feeding on nectar and pollen in some common urban species. (Francis Gilbert, Ecological Entomology (1981) 6, 245 – 262.) Also, this review of the Mouthparts of Flower Visiting Insects (Harald W. Krenn et al, Arthropod Structure & Development (2005) 34, 1–40)
[8 May, 2010. Edmonton, Alberta. Nikon D80 with Tamron 90mm DI lens on KenkoPro 1.4x teleconverter. ISO 200, 1/250 sec. @ f40 (effective aperture) diffused Nikon R1 flash system. Processed in Lightroom 3.3 (cropping, levels, spot removal, sharpening, noise-reduction). Animated gif image made in PhotoScape]