Sunday, July 25, 2010

It's a fly-eat-fly world

This big bristly robber fly has captured a smaller green blowfly. 

Urban wildlife is not all butterflies and baby birds. I found this pair of flies on one of the rustic log fences of the Shakespeare Garden in Central Park.

The robber fly is a female. We can tell by her scary looking sword-shaped “tail” – it’s really an egg laying apparatus called an ovipositor.

Like other robber flies, she is a predator. She has killed the blowfly. After capture, she pierces her victims with a needle-like mouth and injects deadly saliva that paralyzes and digests. Then she sucks out their liquefied insides like bubble tea. The struggle was all over by the time I got there; she was lingering over her meal.

There are many kinds of robber flies -- over 5000 species worldwide, about 1000 species in North America, and about 100 in the Eastern United States. The one in the picture is in the genus Efferia. Robber flies eat other flies, bees, wasps, grasshoppers, dragonflies and spiders. Prey larger than themselves does not deter them.

Robber flies have bristly mustaches that help protect their eyes and faces when their victims put up a fight. 

We may find robber flies disturbing, but they are considered beneficial because of the insects they eat. Just don’t try to pick one up or it might poke you with its needle-like mouth. 

There is more information about urban insects in my book, A Field Guide to Urban Wildlife of North America, which will be published by Stackpole Books in spring 2011. 

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