Excerpts from Jim Conrad's
Naturalist Newsletter

male robber fly, EFFERIA, showing genitals

from the March 27, 2011 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO; limestone bedrock, elevation ~39m (~128ft), ~N18.52°, ~W95.15°
A MACHO ROBBER FLY
On a chilly morning as the sun's first rays slanted in from the east, a robber fly, genus EFFERIA, warmed sunbathing on a leaf next to my compost heap. Since he looked like he'd stay still long enough to get the camera set up I got his picture. It wasn't until his image was on my laptop's screen that I saw something remarkable about him. Can you see what it was in the above picture?

What's that dark, hairy, clublike thing extending from his rear end? I sent the picture to Bea in Ontario who soon came back with the verdict:

The little fellow has what could be called his penis lying out in the sun warming beside the rest of his body. Male robber flies of the genus Efferia are famous for their oversized male genitalia, and now I see why.

Why would Nature create such a thing? On the Internet several pictures show mating robber flies, sometimes with the participants blithely feeding on prey they've just caught, so my impression is that robber fly genitals are big so that during lengthy copulations life can continue as usual without the genitals breaking off, which might happen if they were smaller. Then the question arises of why copulation continues for such a long time among robber flies, and there we can only speculate.

I read that in Arizona 22 species of Efferia of the Pogonias group are known, so who knows how many we have here, and which species this one is? As you might guess, identification in the robber fly genus Efferia is mostly a matter of microscopically examining the species' amazing genitalia.