This is a robber fly, probably an Efferia species. It seems to be ovipositing (laying eggs) on the head of a pale purple cone flower.
What happens after that is open to some speculation. Scientists are not sure how the larvae live or what they eat. There are suggestions that they are external parasites, predators, or plant eaters, or possibly that they have different feeding preferences as different instars (stages between molts).
There are still a lot of unknowns even with relatively common species of animals.