There are over a thousand members of the family in North America alone. Most of them plant their eggs in caterpillars, though I have no idea what species this might use as host. It could also be making use of beetle or grasshopper larvae. Tachinid species tend not to specialize, but be more opportunistic.
Technically, the larva of this fly would be considered a "parasitoid" rather than a parasite, since they kill off their host by eating it from the inside out.
It's not easy being a caterpillar.
And that put me in mind of Jack Handey's take on nature's miracles (via McSweeney's):
In the jungle you come to realize that death is a part of life. The bat eats the moth. Then the giant moth sucks the life out of the bat. Then the monkey eats the giant moth, pulling the wings off first, because he doesn’t like that part. Then the monkey gets a parasite from the moth that slowly eats his brain. It’s all part of the beautiful circle of life.