There are over 150,000 described species of flies (Diptera) in the world, making them one of the most diverse animal orders on the planet. Although they are divided into over 160 families, a full seven percent of all known fly species are members of the massive family Tachinidae. They bear a superficial resemblance to other common flies like house flies (Muscidae) and flesh flies (Sarcophagidae), but have some notable differences. Generally they are a little larger, a little more bee-like in appearance, and have a large number of stiff bristles covering their abdomens.
Tachinid flies are parasitoids of other insects. Females lay their eggs on a host insect, and once an egg hatches the worm-like fly larva eats its way into the host, slowly devouring it from within. Once fat and happy the larva pupates and emerges as an adult. Various Tachinids parasitize a variety of insects and other arthropods, but most commonly attack butterfly and moth caterpillars.
I came across several specific Tachinids (Diptera: Tachinidae: Epalpus signifer) while hiking in Great Smoky Mountains National Park two weeks ago. These adults were feeding on what appeared to be the feces of a raccoon (Carnivora: Procyonidae: Procyon lotor). In spite of adult feeding habits, the larvae of this particular species are parasitoids of pinion moths (Lepidoptera: Noctuidae: Lithophane spp.).