Recently, I at last collected an example of an insect taxon I have long coveted: the Pyrgotidae, a family of acalyptrate flies that (so far as is known) are parasitoids of scarabaeid beetles (Marshall, 2012). The species in question is Pyrgota undata: a large, colorfully patterned insect that bears more than a passing resemblance to a polistine wasp in flight, yet could not be mistaken for anything other than itself when perched.
Female P. undata are known for intercepting airborne "June beetles" (Phyllophaga sp.) and ramming a lone egg between the hapless beetle's momentarily exposed tergites; since these coleopterans are their sole hosts, they can only be found during these beetles' flight period—in Kentucky, mid-April through late May—and during that period when the beetles are most active, namely, at night (although P. undata may rarely be visible during the sunlit hours, as happened in the case of the specimen I personally collected) (Swan & Papp, 1972).
There are merely five genera of pyrgotids in North America (Steyskal, 1978); the family as a whole is distinctive in their lack of ocelli and noticeably protuberant frons (Borror & White, 1970). Of the Nearctic species, P. undata is by far the most likely to be encountered by the layperson.
Borror, D. J. and White, R. E. (1970). A Field Guide to Insects: America North of Mexico. Boston: Houghton Mifflin Company.
Marshall, S. (2012). Flies: the Natural History and Diversity of Diptera. Richmond Hill: Firefly Books Ltd.
Steyskal, G. C. (1978). Synopsis of the North American Pyrgotidae. Proceedings of the Entomological Society of Washington, 80(2), 149-155. Retrieved 5/19/15 from http://biodiversitylibrary.org/page/16381244#page/168/mode/1up
Swan, L. A. and Papp, C. S. (1972). The Common Insects of North America. New York: Harper & Row Publishers.