"Assassin flies are powerfully built, bristly flies with a short, stout proboscis enclosing the sharp, sucking hypopharynx. The name "robber flies" reflects their expert predatory habits; they feed mainly or exclusively on other insects and, as a rule, they wait in ambush and catch their prey in flight.

"Robber fly insects are a mixed blessing to gardeners; if they’re perturbed, they can inflict a painful bite, but they also help rid the garden of harmful pests like grasshoppers, other flies, wasps, leafhoppers, white grubs, and pupating beetles."

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Like other things that eat, they also eliminate internal waste, too.... That brownish drop used to be another insect.