Robber flies are recognizable by the distinctive cleavage between the eyes and the mystax (mustache) on the front of the head. Robber flies can be beneficial by eating mosquitoes and other pest insects. Robber flies can be pests by eating bees and other pollinators or by feeding on parasitoids of pest insects. Over 70 species of robber flies (Family: Asilidae) have been recorded in Michigan. Norman Baker and Roland Fisher (my first college entomology instructor) published a study of Asilidae of Michigan in Great Lakes Entomologist (Vol 8, #2 pp 31-91) in 1975. Their work includes descriptions and keys to species of Michigan Asilidae.
Almost all the Robber Flies in Michigan are native to North America. The European Robber Fly, Dioctria hyalipennis, however, is an immigrant from Europe that arrived on the East Coast and has since become widely distributed. The European Robber Fly perches on or near flowering bushes and preys on pollinators that are attracted to the flowers. Occasionally, large numbers of this species have been collected at a single location. Most of the time, they are few in number and go unnoticed in the background. The one in the picture below was resting on foliage near flowering trees on the shore of Lake Leelanau