A Born Mimic: Toxic Robber Fly Mimics Tarantula Hawk Wasp

by AMNH on

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Mimicry is common in nature, allowing an animal or insect to sneak in closer to prey or to dupe and deflect predators by taking on a more dangerous species’ characteristics.

A fascinating example of the latter can be seen in the robber fly Wyliea mydas, which mimics lethal spider wasps, Pepsis formosa and Pepsis thisbe, known as tarantula hawks.

A close-up photo of a robber fly shows the insect’s black body, head, and legs, and amber-colored wings.
Robber flies like this one mimic the dreaded tarantula hawk wasp, deflecting predators. 
© AMNH/D. Finnin

Like Pepsis formosa and Pepsis thisbe, the robber fly has brilliant orange wings, nature’s “red flag” to other species. Bright colors—red, orange, yellow—tend to be poisonous warning colors throughout the animal world. The scientific name for this kind of warning is aposematism, which from its Latin roots means “signal from a distance.” (This phenomenon and other aspects of poison in nature are explored in the Museum’s special exhibition The Power of Poison, which opens Saturday, November 16).

The robber fly’s deception goes even further. While it has a short proboscis for injecting toxic saliva into prey, it doesn’t sting. Nonetheless, it makes stinging motions, brandishing genitalia at the tip of its abdomen as if it had a stinger.

There are differences, of course, if one looks close. Flies have two wings, while wasps have four. Antennae are short on the fly and long on the wasp.

But the stinging “act” and the orange wings are enough to lead many birds and lizards that might otherwise make a meal of it to bypass the fly for fear of being stung, since tarantula hawks are at the top of the list in the intensity of their sting.

The wasps are also a particularly insidious parasite.

A tarantula hawk wasp drags a tarantula across the sand.
A tarantula hawk wasp drags a tarantula across the sand. 
B. Kimmel/via Wikimedia Commons

The female Pepsis formosa or Pepsis thisbe attacks a tarantula and other large spider by paralyzing it with its sting, dragging it into a burrow or nest, and depositing a single egg on the prey’s abdomen. The larva hatches, bores through the abdomen, and feeds on the host, keeping it alive as long as possible by eating vital organs last.

The robber fly’s wingspan is 2 inches, the tarantula hawk’s, slightly longer. Both are distributed worldwide (in the U.S., mostly in the desert Southwest). Though the mimic fly does a great impersonation of the wasp, the name “robber” is not an allusion to identity theft—it’s a description of how they hunt. They are aerial predators, snatching insects out of the air.

A version of this story appears in the Fall 2013 issue of Rotunda, the Member magazine.