Robber-flies

(Family Asilidae.)

The strong, hairy, active, predatory flies, known as robber-flies, form this group. They are very numerous and are always conspicuous, flying with a darting motion and preying upon many different kinds of insects. They are, as a rule, rather slender, but extremely strong, and are furnished with a large, tapering, hard beak, enclosing a sharp lancet which is thrust out and cuts a severe wound in the body of the insect captured. The tip of the beak is bearded with stiff bristles which hold it securely in the wound into which it is crowded. Fitch says, "These flies are inhuman murderers. They are savages of the insect world, putting their captives to death with merciless cruelty. Their large eyes, divided into such a multitude of facets, probably give them the most acute and accurate vision for espying and seizing their prey; and their long, stout legs, their bearded and bristly head, their whole aspect indicates them to be of a predatory and ferocious character. Like the hawk, they swoop upon their prey, and grasping it securely between their forefeet they violently bear it away." Nearly all of their victims are captured on the wing, and any flying insect is liable to be caught by them--other flies, bees, beetles, moths, butterflies, grasshoppers, and even members of their own species, so that they are true cannibals. Just as with the praying Mantis, or rearhorse, the female frequently resents the caresses of the male, and grasps him and eats him. They will also feed upon caterpillars, but rarely. Persons engaged in bee culture especially fear these robber-flies, which are known rather generally in this country as bee-killers. One of Dr. Fitch's correspondents sent in an interesting account of the damage done to his apiary. The robber-fly captured bees by making rapid dashes catching them on the wing, then wrapping its legs about the bee, and pressing it tightly to its own body it immediately sought a bush or tall weed upon which to alight and devour its prey, piercing a hole in the body and sucking out the fluids and soft internal viscera, leaving only the hard outer skin. Upon the ground beneath some favorable perch for the fly near the apiary hundreds of thse shells of bees were found accumulated in a single day. The correspondent thought that it was through the work of the robber flies that during certain seasons in a bee raising region in New York not a single hive threw off a swarm. The beak of a robber-fly is so strong that it can pierce the skin of a human being, but fortunately none of these creatures has yet acquired the habit of feeding upon warm-blooded animals.

Some robber-flies are very delicate and slender, as in Leptogaster, some of them being almost as slender as midges, upon which they probably feed, and looking half starved, in spite of their voracity. The colors of the robber flies are variable, but nearly all are very hairy or bristly or spiny. In this family some good cases of what is called "aggressive mimicry" are seen, aggressive mimicry meaning a resemblance of a predatory insect to the insects upon which it feeds, thus facilitating the capture of its prey. We should not fear grizzly bears if they looked like harmless, peaceable human beings. Thus the robber-fly known as Deromyia annulata Bigot, looks like the common wasp Polistes metricus Say; while some of the flies of the genera Dasyllis and Mallophora resemble bumblebees. In some of the latter the hind shanks are modified so as to look very much like the pollen-bearing hind legs of the bumblebees. This curious structural modification can be of no service to the fly except in increasing its resemblance to the bees. Then also, as another illustration, one of the robber-flies of the genus Laphria resembles a big wasp of the genus Vespa.

The larvae of the robber-flies much resemble the larvae of the gad-flies, although the adults are so widely different. They live in the earth, and in decaying wood, and prey upon the larvae of wood-boring beetles. In the ground they have been known to feed upon the eggs of grasshoppers. Harris has described the early stages of one species (Asilus sericeus) which he thought fed in the larval stage upon the roots of rhubarb. Harris was seldom mistaken, but it is safe to say that as a rule the larvae are carnivorous. The pupae are bristly, and have the head and the segments of the abdomen provided with spines which assist them in making their way out of the ground or the decaying logs which they inhabit.

The group is a very large one and comprises about three thousand described species, of which four hundred or more inhabit this country. The detailed life history of some robber-fly is a great desideratum.