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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Wasp or fly? Stilt-legged flies, mimetic Micropezidae


With front legs outstretched a stilt-legged fly does its best imitation of a wasp.


Last summer while sitting on the porch of a log cabin in West Virginia, I and my fellow bug geeks spotted what appeared to be a medium sized parasitic wasp inspecting some morsels of food left behind from a midday repast. Upon closer inspection, and noting the presence of only a single pair of wings rather than the standard two pairs found on most wasps, this gangly-legged insect turned out to be a stilt-legged fly. Unusually long and skinny legs are the hallmark of this family. As we watched, its rather robust proboscis pulsed across a tidbit of cheese apparently lapping up yummy liquid nutrients.

Micropezids are somewhat uncommon flies often mistaken for wasps or ants by virtue of their coloration and hymenopteran-like behaviors. I encountered several interesting stilt-legged flies on recent journeys to Central and South America where these mimetic behaviors were on full display.  One dashing fly sported white-tipped forelegs that were pretty much dead-ringers for white-tipped antennae found on some species of wasps. To complete the illusion, the forelegs were periodically outstretched in front of the fly, giving the impression that this was indeed an insect with a stinger rather than simply a juicy fly. A bit further down the trail I stumbled on a second species. By executing a backward moon-walk with elevated hind legs and an upturned abdomen, this little fly looked for all the world like one of the rainforest ants notorious for their powerful stings. Adult stilt legged flies are known to be predators and also feed on decomposing organic matter, including dung. Larvae are found in the soil, rotting logs, and excrement. Mimetic coloration and behaviors may deceive predators and help to ensure survival of the strange stilt-legged fly.

Waving the white-tipped forelegs may mimic the movements and color patterns of some species of wasps. Whether it’s snacking on a morsel of cheese, walking across the surface of a leaf, or depositing eggs in rotting wood, stilt-legged flies regularly keep their forelegs out front and dancing. Watch as another species of stilt-legged fly does its best ant imitation by walking backward, hind legs outstretched like antennae, and abdomen bobbing like the head of an ant.


Bug of the Week thanks fellow bug enthusiasts Larry, Sandy, Nan, and Paula for the inspiration behind this episode.