Not all flies are "bad," this robber fly is dining on a bot fly. Summer is waning and it's time for the adult robbers to show their rapacious heads. They have been growing up in dung heaps, soil and leaf debris, and compost piles, devouring anything that tasted good.
Fortunately, these robbers are not as big as house cats, if they were, you wouldn't be safe walking or riding your bikes on forest and desert trails. In fact, most people don't even notice them as they go speeding by in pursuit of a tasty meal.
Moreover, they are fast! I sort of look at these robbers as Nature's F-16 Viper jets. What I'm talking about are Robber Flies, the ultimate flying predator of the insect world. Once they start after a meal, it is dead meat!
The photo above shows the "good" part of the world of Nature, "Good" being in human terms. The hapless insect that robber fly is sucking dry is a bot fly, which, as those with livestock know, is a bothersome insect.
Bot flies grow under the skin of mammals, (humans included) feeding on blood and tissue of their host. When the grub emerges to metamorphose into an adult, it leaves a gaping infected hole in the host's skin. Not a pleasant sight...
Robber flies (and bot flies) are "true flies;" that is, they have two wings, while all other flying insects have four. To make up for the missing wings, flies have two "balancers" (known to the fly experts as "haltares") that keep the insect stable in flight, acting like gyroscopes. The balancers also provide flies with agility in flight that is equal to hummingbirds. Anyone who has attempted to swat a fly and missed knows how quickly a fly can change direction.
That is also a wonderful tactical advantage for the robber fly. They usually perch on a stick, watching for a tasty meal to appear, and with those huge, multifaceted eyes, not much escapes their attention. Once a likely victim is spotted, the robber fly blasts off like an F-16 in pursuit of the target. No manner of avoidance on the part of the intended target works, the conclusion of the chase is inevitable. The robber fly overtakes its victim, grasps it with powerful middle legs and with one quick jab, plunges its sharp proboscis (mouthpart) into the body of its prey.
The victim may struggle to escape, but it is futile. Once the short, strong proboscis is jabbed into the victim, the robber fly injects saliva containing neurotoxic and proteolytic enzymes, which paralyzes and digests the insides, making further struggling impossible. Then the fly sucks up the liquefied meal, much as we slurp up ice cream soda through a straw.
In one scientific camp, it is said there are over 7,000 species of robber flies worldwide, while in another camp, they limit them to about 6,750. However, both camps agree that there are about 1,000 species in North America (with over 100 species in Florida alone).
As with most predators in the Wild, robber flies are also opportunists. If there are lots of grasshoppers around that will be the target of the day. When the grasshoppers run out they will then turn to dragonflies, houseflies, flower beetles, butterflies, crab spiders or whatever.
Most of the US robber fly species fall into the Family, Asilidae (ah-Sill-li-dee). In the 1800's, a dipterist (fly expert) by the name of Loew was perhaps the most influential contributor of information on the study of robber flies, describing several species and more than 80 genera. Later dipterists in the 1900s became specialists of robber flies in particular locales, most notably Curran and Bromley in North America.
Way back in the '40s the hero of most naturalists and wildlife biologists, Professor Aldo Leopold, author of A Sand County Almanac, and other wonderful publications, said, "In time, wildlife research will become wildlife recreation." And that's the way it is for people like you and me. We pursue the robber fly today for the sheer joy of observing it doing what it has to do to fulfill Darwin's so-called "theory of evolution."
As I watch the comings and goings of the world of nature around us, it appears to me that Darwin's "theory" is, in reality, a "fact." In order to succeed and insure the survival of the species, robber flies in my neighborhood have begun perching very close to my bee hives, and guess what the targets are... With an unlimited food source - new bees emerging from the brooder box every 28 days or so - the robber flies probably think they have died and gone to heaven; no wonder they're also known as "bee killers."