Monday, August 24, 2009

Not a Wasp, But a Fly That Looks Like One

On Saturday, I posted a series of photos of insects that I found recently in the garden. As it turns out, I identified one of them incorrectly. The photo below shows two wasp-like insects, both of which I originally identified as wasps.

The insect on the left is not actually a wasp but a mimic fly, in the family Conopidae. This individual is very similar the Kaufman guide's illustrations for the genus Physocephala. (Thanks to Anita Gould, whose post on Flickr brought the mistake to my attention.) Flies in this family, known as the thick-headed flies, are often found around flowers and mimic stinging insects, possibly to fool predators into seeking easier prey. In this case, the ruse was enough to fool me.

Despite its appearance, this is clearly a fly. The telltale sign is the single set of wings attached to the thorax. The shape of the antennae and hind femurs are also clues. You can see the single set of wings in the photo below.

Bees and wasps, meanwhile, have two sets of wings. The photo below, a Philanthus wasp, does not have great detail. However, it is just possible to see both a forewing and a hindwing attached to the right side of its thorax.

Like many wasps, flies in the family Conopidae parasitize other insects, in this case solitary bees and wasps. According to the Kaufman guide, female Conopids drive flying bees to the ground and hold them while they force an egg into the host's abdomen.