Lurking in sunlit forest clearings and along mountain trails and country lanes, robber flies wreak havoc; or in the open fields where wild flowers attract bees and butterflies, chase and kill them. They are very active creatures and live at the expense of other insects. They grab them with their strong legs and claws, and then drive their beaks firmly into the bodies of their victims, sucking the blood with great voracity. Robber flies belong to one of the largest families of the great order Diptera or two-winged flies. Many of them are large, the largest being nearly two inches long. They are usually hairy and some of the species are quite robust, resembling bumblebees in shape and color; others are elongated with slender bodies.
Viewed from the forehead, the thief’s head is broad, the compound eyes are prominent, and the rest of the face is hairy and bearded. The proboscis or beak is robust and strong, and is formed for piercing and sucking. Strong in flight, the two wings are long and narrow, while the legs, which are spiny and have strong claws on the toes, are used to grasp their loot and to support their body when resting. These predatory insects rest on the ground or on the foliage of plants growing in open, sunny places. Here they lie in wait for their prey, and when a victim appears in the form of some other insect, they rise into the air with a loud buzzing, catching it in their wing. The unfortunate insect, once captured by the powerful talons of a robber fly, is unable to escape.
They will attack most insects, and are even bloodthirsty enough to catch and eat their own kind. They have often become a nuisance by making their air in the vicinity of an apiary, where they kill bees. One of the largest species was observed during the summer capturing a “locust” or cicada. The robber fly attacked the cicada on the wing about twenty feet above the ground, and the pair descended in a whirlwind. In this case, the loot was too bulky to carry to some convenient roost, as is often the case. Fortunately, robber flies never attack man or animals, though if caught carelessly they will stick their lancets into flesh.
The larvae or “worms” that hatch from the eggs laid by these flies are also carnivorous. Some of them live on the ground, where they search for food among decaying vegetation; others make their home in rotting logs or under the loose bark of dying trees, where they hunt and feed on other soft-bodied insects. If we follow the fate of one of these larvae or “maggots”, we will find that after consuming enough food and overwintering, it will go through the usual transformation, finally emerging from the pupal case as a perfect robber fly and a true thief. .