|Long-Tailed Dance Fly - Rhamphomyia longicauda|
Family Empididae -- balloon flies, dance flies
Live adult dance flies photographed in the wild at Winfield, Illinois. Size: Female: 11mm Male: 10mm
Species epithet longicauda, Latin "long tail." Dance flies are known for their mating swarms which sometimes occur during daylight, but most often at dusk when they are very difficult to see.
Male dance flies of some species are known to capture smaller insects and wrap them in silk, then fly about offering the prey to receptive females in the swarm. They have even been known to offer empty balls of silk in an effort to deceive females into mating with them. It is not known how often this practice is successful, however, one would think evolutionary pressures would have eliminated the deceptive strains were it not a viable reproduction tactic.
Females in the swarm are similarly engaged in deception: they pump up air sacs in the abdomen, ostensibly in an effort to fool the males into thinking they are bulging with ripe eggs. The fringed legs are said to enhance the effect. 
Teneral female in the process of pumping up her wings; ovipositor is visible upper left of center.
The American midwest was seeing a population explosion of these flies this spring and early summer of 2005. Adults of both sexes dance in swarms of several dozen flies at twilight; their dark colors make them almost impossible to see at this time of day. I had to capture several of them with my hand before I could tell what they were. They fly up and down and in a circle about a foot above the ground.
I then noticed them flying about in the daytime in heavily shaded areas of deciduous forest. My Audubon Field guide says adults prey on small insects, and larvae live in water or decaying vegetation.
Lateral view shows wings longer than abdomen
Female in flight reveals how the legs are held
Habitat: Understory in wet deciduous woods, often along ponds or streams. Adults rest on foliage during the day; their courtship flights begin at late dusk, when it's almost impossible to see their swarm. They fly in vertically-elongated ovals about 2-3 feet high, near the ground. I have seen swarms with perhaps 3 dozen females. Males meanwhile hunt small insects as "nuptial gifts" they give the female in exchange for sex. 
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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