It’s a pity they aren’t better appreciated, because North America is home to about 1,000 of the more than 7,000 species known world-wide, so there are plenty of them to see—and they are well worth a closer look. They range from 0.1-2 inches (3-50 mm) in length, so admittedly some species can be pretty hard to find, and they like open, dry, sunny habitats. Like all true flies (Diptera), they have a single pair of wings, with the front wings used for flight and the rear pair reduced to small clublike balancing structures called halteres. One obvious feature of robber flies which is unique to this family, however, is their comically hairy faces, which give them the appearance of sporting a stylish ‘stache or bushy goatee. The official name for this fuzzy feature is the mystax, derived from the Greek mystakos meaning mustache or upper lip, and it is thought to protect their heads from the frantic movements of struggling prey. The top of their head is indented between widely separated compound eyes, and many have a stout thorax with a slender tapering abdomen. Some are wasp or bumble bee mimics, with striped and/or furry black and yellow bodies. “Raptor flies” might be a better name for this group, given their prodigious hunting abilities, but robber flies harbor no animosity towards humans and will not bite if handled gently. In fact, they land on people fairly readily, and most of my asilid photos have been taken when one landed on me to leisurely consume its large, recently-captured prey.
Robber flies are visual predators, but recent research shows just how amazing their vision is. A team of researchers studied one of the smaller Asilidae, Holocephala fusca, which clocks in at a mere 6 mm long (Wardill et al. 2017, A novel interception strategy in a miniature robber fly with extreme visual acuity, Current Biology 27: 1-6). This tiny fly can spot potential prey more than 0.5 m away in less than half a second, which is about the same ratio as a human seeing a moving sandwich across a soccer field. Like all adult insects, robber flies have compound eyes made up of thousands of individual lenses called ommatidia. Robber flies have lenses in a range of different sizes, with larger lenses providing better vision; some lenses in the robber fly’s eyes are as large as those in the eyes of a dragonfly, an insect about 10 times larger. A dragonfly can have large lenses across its entire visual field, but a robber fly’s head simply isn’t big enough to accommodate the quantity of large lenses needed for this degree of visual acuity. In a neat evolutionary solution, the eyes of a robber fly are arranged with more of the larger lenses in the center of the field of vision and smaller lenses towards the outside of the eye, providing high acuity in a small space.
When a robber fly spots its prey, it takes off on a fast interception course using maximum acceleration. However, it is usually chasing a moving target that may alter its own speed and direction, so the hunter must be able to adjust course once it is in closer range to achieve a successful capture. In the studies on Holocephala, flies pursuing “prey” (a moving bead) used a constant approach speed when first zooming towards their target, but adjusted heading and speed once they were within about 29 cm (11.5 in.) of their target. The authors describe this as a “lock-on strategy”, and state that it has not been described in any other flying animal. They hypothesize that what flies lose in speed during this lock-on, they gain in increased likelihood of capture, making it an energetically worthwhile tradeoff. They further note that this strategy may be of future use in designing “bioinspired guidance systems” for miniature aerial vehicles, an area in which dragonflies have also provided inspiration.
If any of this has inspired you to find out more about the amazing robber fly, check out the web site of Asilidae biologist Fritz Geller-Grimm, which also has links to other researchers, some of whom have written regional field guides. As the British pathologist Edward Halford Ross wrote, “Each fly is king of his own country,” so the next time you see a robber fly zooming towards an intended victim, give it a small salute.