You may well have seen a robber fly (also known as an assassin fly) and not recognized it as such. Big eyes, pointed mouthparts, a hairy face and a fondness for insects are robber fly characteristics to look for.
Robber flies are among our largest flies and are predatorial ambushers. They tend to perch for minutes at a time on the tips of leaves, or other sunny viewpoints, where they keep a lookout for unsuspecting prey. Once they spot an insect they lose no time in darting after it. With their beak-like mouthparts they spear the insect and inject a mixture of nerve poisons and enzymes that liquefy the tissues of their victim. They then drink the innards of their prey.
Note that there is a prominent “beard” or tuft of bristles (mystax) in the front of the pictured robber fly’s face. All robber flies have this tuft which serves to protect the face and eyes of the predator from the struggling prey as it dines.
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Milkweed leaves make excellent platforms for all kinds of insects, particularly those such as dragonflies and robber flies, which sit, wait and watch, surveying the landscape for prey to ambush. This robber fly could quite easily be mistaken for a bumblebee. However, the short, straight antennae and the presence of only two wings (instead of four) tell you it’s in the order Diptera (true flies). The pointed, stout proboscis, bearded face, fleshy feet and long, tapering abdomen narrow it down to a species of robber fly. Robber flies in the genus Laphria resemble bumblebees – they are typically quite hairy, with black bodies and yellow stripes on their abdomens. Like other species of robber flies, they hunt by perching and snagging prey such as other robber flies, bees, wasps or beetles. They often return to their perch, inject the prey with enzymes that dissolve its innards, and then have a long drink. Why mimic a bumblebee? To deceive unsuspecting honeybees, wasps and other insects that would make a good meal.