These delightful flies seem to be gigantic eyes to which a body is appended. Their head is hemispherical or almost spherical, with the eyes taking up much of the head's visible surface. Big-headed Flies are also a delightful family because of their relative scarcity—it is a delight to find one, though it does not happen often.
A third and final element of delight may be mentioned, and that is the fascinating life history of Big-headed Flies. After mating, females fly in search of Leafhoppers or Delphacid Planthoppers. The female fly lays an egg in the Homopteran host's body, and there the egg hatches and develops, feeding on the host until the host's entire body cavity is filled. The larva then escapes the host, falling to the ground and pupating in the soil or leaf litter (Hardy, writing in McAlpine, 1987).
While the known hosts of Pipunculid flies have been Homopteran hoppers, the life histories in this family are still largely unknown. In 2005 researchers published a startling discovery about the life history of at least one Big-headed Fly species; more information is on our Nephrocerus sp. page.
As far as identification goes, all the emphasis on the size of the eyes can be misleading, since members of other fly families can have huge eyes that dominate the head (Platypezidae, for example), and on the other hand even the males of some Big-headed Fly genera have eyes that don't touch. Some other traits to watch for:
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Insects of West Virginia