Rhagionidae or "snipe flies" includes two groups that have sometimes been assigned to the families Xylophagidae and Coienomyidae. They are predaceous in both the adult and larval stages. Adults are consistently predaceous, their prey being principally adults of other Diptera, and several species have been reported to have developed bloodsucking behavior. Most are abundant only in very moist habitats, but some occur under arid conditions. The larvae are usually predaceous, preying on a variety of insects and smaller animals (Clausen 1940/62).
The two genera, Lampromyia and Vermileo have been thoroughly studied in Europe. Vermileo vermileo L. was investigated very early in the history of entomology by DeReaumur (1753) and Degeer (1776). Larvae of this and species of similar behavior are called "worm lions" to distinguish them from ant lions of the Neuroptera, with which they have some similarity. Larvae construct pits in the dust and sand at the bases of cliffs and other similar locations. These pits are conically shaped, and the Vermileo larva lies on its dorsum at the bottom of the pit where it encircles any suitable prey that falls within reach, dragging it underneath the sand. Ants are one of the principal foods. The behavior of V. comstocki Wheeler was discussed by Wheeler (1930). This species inhabits glacial silt and dust in the mountains of western North America. Eggs are laid in masses of 50 or more, adhering together in rows in the dust or silt. They hatch in ca. 8 days. Young larvae are gregarious, but later instars are solitary. The larva lies in the inverted position in the pit, with its anterior end across the bottom. Prey consists principally of a common ant species, Formica fusca L. Engle (1929) reported on behavior in Lampromyia sericea Westw. and Wheeler (1930) on several species of Lampromyia. Larvae of African L. pallida Macq. differs in behavior from others of the genus and from Vermileo by holding its body in a vertical position, with the anterior end coiled at the bottom of the pit where it awaits its prey.
Species of the genus Chrysopilus that have been studied are quite different in their larval habits and host preferences from those previously discussed. Chrysopilus nubeculus Macq. is predaceous in the egg capsules of Algerian locusts, and C. ferruginosus Wied. lives in decaying plant roots or tissues in the East Indies and Philippines. The latter species is a predator on the larvae of the banana weevil, Cosmopolites sordidus Germ., and the sugarcane beetle borer, Rhobdocnemis obscura Bsd.. it was introduced into several areas in efforts at biological control. Froggatt (1928) reported that the eggs were deposited in clusters of 16-60 in the borer holes in slightly decayed stem tissue. Young larvae found in decaying stems that contained no weevil larvae were believed to be phytophagous.
In North America Atherix variegata Wlk. is an aquatic species of considerable abundance. Large egg masses and the remains of many flies have found on the underside of a Connecticut bridge by Britton (1936). The largest of the masses covered an area of ca. 50 sq-ft., and was at least one-half inch deep. Eggs hatch in ca. 6 days, and young larvae drop into water, where they feed on various soft bodied insects and other organisms. Some species of Symphoromyia also have aquatic larvae (Clausen 1940/62).