Flies in the family Tachinidae (Parasitic Flies), commonly called tachinids, make up the second largest family of flies in the world, with about 1,300 species in North America and more than 8,000 species worldwide (and perhaps only half of all species are known). Tachinid larvae are parasitoids of other arthropods including insects, centipedes, spiders, and scorpions. They are important biological controls for several agricultural pests.

Tachinids vary widely in body shape, but many have a distinctly bristly abdomen and characteristically large antennae.

The female often deposits eggs directly on—or inside—the host, but some hosts swallow the tachinid eggs or larvae while eating leaves on which the eggs were laid. Eating their way out, the tachinid larvae kill the host, which is why they’re called parasitoids instead of parasites.

Some tachinid species incubate the eggs internally, laying eggs nearly ready to hatch. In other species, the eggs hatch within the female, who gives birth to live young.