Tachinidae is a large and rather variable family of true flies within the insect order Diptera, with more than 8,200 known species and many more to be discovered. There are over 1300 species in North America. Insects in this family are commonly called tachina flies or simply tachinids. As far as is known, they all are parasitoids, or occasionally parasites, of Arthropoda.
Larvae (maggots) of most members of this family are parasitoids (they develop inside a living host, ultimately killing it), and a few are parasitic (they do not kill the host). They are endoparasites (internal parasites) of caterpillars of butterflies and moths, adult and larval beetles, sawfly larvae, various types of true bugs and grasshoppers, and (rarely) centipedes, with usually only one grub. Only a few species are restricted to one host species. As such, many are important natural enemies of major pests. Some species are being used in biological pest control. Many important pests are suppressed by tachinids. Many species of tachinid flies have been introduced into North America from their native lands as biocontrols to suppress populations of alien pests.
Reproductive strategies vary greatly between species. The female may lay white oval eggs on the skin of the host insect, or insert eggs into the host's body, or leave them in the host's environment, as for example on leaves, where the host will ingest them. Some tachinids that are parasitoids of stem-boring caterpillars deposit eggs outside the host's burrow, letting the larvae do the work of finding the host itself. In other species, the maggots use an ambush technique, waiting for the host to pass and then attacking it and burrowing into its body. The larvae feed on the host tissues.
Tachinid flies are extremely varied in appearance. Adult flies may be brilliantly colored and then resemble blow-flies (family Calliphoridae), or rather drab, and then resemble house flies. But tachinid flies are more bristly and more robust. They have three-segmented antennae, a diagnostically prominent postscutellum bulging beneath the scutellum (a segment of the mesonotum), and bare (sometimes plumose) arista. The calypters (small flaps above the halteres) are usually very large. Their fourth long vein bends away sharply.
Adult flies feed on flowers and nectar from aphids and scale insects. As many species typically feed on pollen, they can be important pollinators of some plants, especially at higher elevations in mountains where bees are relatively few.
The taxonomy of this family presents many difficulties. It is largely based on morphological characters of the adult flies, but also on reproductive habits and on the immature stage.