Tachinid flies are mostly robust flies with spiny abdomens. Adult tachinid flies prefer sucking up sweet nectar from flowers, but larvae prefer protein. Of the 1,345 species of tachinid flies in North America, larvae are almost exclusively internal parasitoids of other insects, especially caterpillars. Typically, the females lay eggs on the host, and the larvae burrow inside. They feed within the host, avoiding vital organs until the last stage of their own development (usually killing their host). For this genus, hosts vary with the different species and include the caterpillars of a variety of moth species, walking sticks (not present in Montana), larval leaf beetles, sawflies, and bumblebees. 
Photo by Glenn Marangelo on 9/12/20 in Missoula, MT