Gathering pollen. This is a female. Quoting <http://www.flickr.com/photos/sankax/5917256258/>: "Syrphid Fly (Toxomerus marginatus)...This is a female. You can tell because on the top of the head the eyes have a space between them. Male's eyes touch on the top of their heads.

"Adult syrphid flies feed on pollen and nectar, while in the larval stage they feed on insects. Larvae of predaceous species feed on aphids and other soft-bodied insects and play an important role in suppressing populations of phytophagous insects. Larvae move along plant surfaces, lifting their heads to grope for prey, seizing them and sucking them dry and discarding the skins. A single syrphid larva can consume hundreds of aphids in a month."

Pierre Martineau: "The data regarding the Syrphidae diet sound accurate. Insects being so diverse, there are always specific exceptions, and it is always best to find data confirming whether a group's habits apply to an individual species. We are in luck here: the paper at
<http://web.entomology.cornell.edu/shelton/diamondback-moth/pdf/85papers/1985DBM38.pdf> on page 410 reports observing Toxomerus (i.e. Taxomerus) marginatus preying on aphid colonies."