It looks like a bee. But it's a fly. It's a bee fly

By Jeff Mitton

I had hiked to the top of Sugarloaf Mountain to enjoy the day and perhaps get some photos.

I was sitting there, gazing at the divide, when a noisy bee flew up and hovered at my eye level, just three feet in front of me. Then I noticed that it had a long black projection from its head that reminded me of the tusk of a narwhale. I concluded that this was neither a bee nor a narwhale and later learned that it was bee fly.

It hovered for about 30 seconds, then flew off toward some low flowers. I grabbed my camera and followed, but this buzzing, hovering insect was always in motion -- that day and on subsequent days, I failed to get a photo that I was willing to share.

Last week my Ph.D. student Scott Ferrenberg and I were visiting a field site a few hundred yards west of Sugarloaf Mountain. Early on a cloudy and cool morning, Scott, a keen-eyed entomologist, pointed out bumble bees and bee flies sitting motionless on stalks of mullein (Verbascum thapsus) or on the drying heads of blanketflower (Gaillardia aristata). I took advantage of their chilled somnolence to get some photos.

While I was shooting the bumble bees, I was puzzled by the fact that some had not yet stirred while others were already moving industriously from flower to flower. Carol Kearns, a member of my department who literally wrote the book on Colorado bumble bees, told me that the sleepy ones were probably males, slothful drones, while those bustling through the flowers were females.

The family Bombyllidae, which contains the bee flies, is distributed worldwide; 4,700 species have been described and many more are waiting to be discovered and described. Individual species tend to be widespread geographically but not particularly abundant in any locality, and consequently we know rather little about most of them. I am taking an educated guess that this one is the large bee fly, Bombylius major.

The large bee fly is very fuzzy, and it buzzes as it flies or hovers, so it is easily mistaken for a bee. But the long legs and the tusk-like mouth parts identify it as a bee fly.

Adult bee flies eat pollen and nectar; their distinctive mouth parts are modified for sucking nectar from flowers with long corollas, particularly primroses. They rarely perch, but hover while feeding, steadying themselves with their front legs on petals. This continual activity makes them difficult to photograph.

Females do not make nests, but deposit eggs in the sand at the edges of burrows used as nests by solitary wasps and bees. They stir the eggs with the sand and then use their legs to flick the eggs toward the eggs and larvae in the nest. When the eggs hatch, the larval bee flies parasitize the eggs or larvae.

While some parasitoids are specialists on a single species, bee flies are opportunistic generalists. In addition to wasps and bees, bee flies parasitize other flies, beetles and antlions.

The large bee fly is a mimic of bees, perhaps for protection from predators, but more likely so they can get close to the burrows of solitary bees without triggering a defensive response.

Bee flies are harmless to humans. They cannot bite because their mouth parts are modified for sucking fluids, and they can't sting because flies don't have stingers. So you can approach and observe them closely without fear of retribution.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

September 2013