Stilt-Legged Fly - Taeniaptera trivittata
FamilyMicropezidae - Stilt-Legged Flies

Live adult flies photographed & filmed at Black Partridge Preserve, Cook County, Illinois, USA.
Stilt-Legged Fly
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I would have called this the "semaphore" fly, because it constantly waves its front legs around as if signaling with flags. I think the action is a sexual display roughly akin to those of the birds of paradise. (except both fly genders do it.) Some entomologists think these flies are mimicking the jointed antennae of the vesip wasps, as do some Syrphid flies.

Similarly, male flies in the family Dolichopodidae, commonly called "long-legged" flies (right) have "mittens" on their front legs, which they also wave about. These mittens are secondary sexual characteristics and attract females.

Family Micropezidae consists of about 40 genera with 475 described species worldwide. I have always found these gentle creatures in wooded settings most often near flowing water.

Dolichopodidae - longlegged fly
Long-Legged Fly, Family Dolichopodidae
A stilt-legged fly signals its sexual availability

Stilt-Legged Fly

References
  1. Bugguide.net Micropezidae
  2. Bugguide.net, "Taeniaptera trivittata"
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Flies of North America - Order Diptera. Flies are prevalent in virtually all habitats, with over 16,000 species in North America. Flies can be distinguished from all other insects in that they only have one pair of normal wings. The other pair has evolved into small ball-like structures called halteres. Most flies have compound eyes and mouthparts adapted for piercing, lapping or sucking fluids.
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