Signal Flies - Family Platystomatidae
Noted for their elaborate mouthparts, these flies closely resemble those in the family Ulidiidae.
Live adult signal flies photographed in the wild at Ogle and DuPage Counties, Illinois, USA.
Signal Fly Platystoma seminationis
Platystoma seminationis was very abundant in tall grass in a weedy field near the west branch of the DuPage River in Illinois. Size = 6mm

Flies in the family Platystomatidae are commonly called "signal" flies, after their habit of constant motion, as if sending signals. [1] They are often confused with flies in the family Ulidiidae, the so-called picture-winged flies, which they very closely resemble.

There are 41 described species in 4 genera in North America. They range in size from 3 to 12mm.

Signal Fly Rivellia sp.
Signal Fly Rivellia sp. Size = 4mm.
Signal Fly Rivellia sp.
This tiny fly was found in Ogle County, Illinois, near the Rock River
References
  1. Bugguide.net, Family Platystomatidae - Signal Flies
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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. Most flies have compound eyes and mouthparts adapted for piercing, lapping or sucking fluids.
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