Tachinid flies are parasites, their larvae developing inside other insects. (In a few cases their larvae develop inside other kinds of arthropods.) In some species of Tachinid, the adult female has a piercing ovipositor and lays her eggs inside the body cavity of the host insect. In other cases, such as the one illustrated here, the eggs are cemented to the integument, and the newly hatched Tachinid larvae will burrow their way into the host’s body.
The host shown here is a Hickory Horned Devil, the caterpillar of Citheronia regalis. Among the most common parasites of the Hickory Horned Devil are species of Tachinid fly in the genus Belvosia. At least one species of Winthemia, another Tachinid fly, also parasitize Hickory Horned Devils.
The Hickory Horned Devil, by the way, is one of the largest caterpillars one is likely to encounter in West Virginia or elsewhere in the region. It is about the size of a hot dog.