The Family Tachinidae is a large one, with about 1300 species in North America, and in fact it is the second largest family in the Order Diptera (the true flies). The tachinids are almost exclusively "parasitoids", which means they deposit eggs on the surface of the body of other insects. The eggs hatch and the developing larvae bore into the body of the host, feeding on the internal organs, saving the vital organs for last. The developing fly larvae typically exit the body of the host to pupate, and this generally results in the death of the host (like an alien jumping out of your chest). In some species of tachinids, the "eggs" actually contain relatively well-developed larvae, which feed and grow rapidly in the body of the host. In the field cricket, development of the tachinid larva in the cricket host takes about a week, so that a parasitized calling male cricket's days are numbered, and in fact he will be debilitated by the parasitic fly larva well before he is killed by the exit of the larva.

Tachinids parasitize a wide variety of insects, but most prefer beetle grubs, caterpillars (larvae of lepidopterans), and sawflies (a kind of hymenopteran). Other kinds of insects that are parasitized by tachinids include hemipterans (true bugs) and of course orthopterans like the field cricket. Tachinids are important as "biological controls" of various agricultural pest species, and several species of tachinids have been introduced into this country in attempts to control introduced insect pests.

Flies in the related family Oestridae (the bot flies) are also parasitoids, but these insects parasitize vertebrates rather than insects. There are a number of species in North America that parasitize various rodents and rabbits, and there are tropical species that parasitize livestock and people. Their preferred location is under the skin of the neck, where the larvae grow to a few centimeters in length. When they are ready to pupate the larvae bore through the skin, leaving a hole which can potentially become infected. Native peoples in Central and South America have folk remedies for this parasite, which involve using a "poultice" to drive the parasite out of the patient's body.