The Tachinid Flies are a very large family of flies with over 1000 species noted in North America.
Eggs of Tachinid Flies are laid in (injected) or on (penetrate) the surface of other insects -- especially the caterpillars of butterflies and moths. The fly larvae feed on the internal structures of the caterpillars ultimately killing it. Adult flies are not parasitic, either not eating at all or utilizing other energy sources.
While they are important in controlling many insect pests, they tend to be indiscriminate in terms of their hosts. Thus, they also parasitize the caterpillrs of butterflies such as the Monarch and Silk Moths.
