Dirk J. Stevenson
One humid May morning, on an aeolian dune above a blackwater stream (aptly named Alligator Creek), I lay on my belly, fully outstretched, on a sizeable butter-yellow spoil of sand − the apron of a large, active gopher tortoise burrow. Dozens of other burrows, including some where I have observed indigo snakes, Florida pine snakes, and gopher frogs, pock the nearby landscape. The mellifluous note-then-trill of a Bachman’s sparrow makes it clear I am on his turf.
But this isn’t a herp mission. Extending my arm into the burrow tunnel I scoop moist sand from the burrow floor. Straining to reach as deep into the reptile cave as possible, I use a mammoth-sized (think long-haul trucker) plastic coffee mug secured to the end a mop handle (with a dozen long zip-ties) to bring several loads of heavy wet sand to the surface. My ostensible, and I daresay noble, goal: To collect fresh tortoise dookie from the interior of a gopher hole − which I will carefully sift, with primate fingers, in search of odd obligate coleopteran commensals (i.e., the staphylinid, scarab and histerid beetles that recycle tortoise dung or prey on those that do) that most sandhill enthusiasts only read about in books.
My cheeks and chin bearded with sweat and sand, I withdraw my scooper. A single fly follows. It’s not one of the drab anthomyiid flies, Eutrichota gopheri, that populate, in large numbers, most active burrows (like a puff of smoke, a small congress of them emerged from the burrow in unison when I slapped my backpack on the burrow roof). No, this much larger, and solitary, fellow, is a rich golden-brown. He flew from the burrow, landed on my snake boot, took a few pensive seconds to contemplate the bright outdoor world, then, as if on a string, flew directly back into Casa Gopherus. After years of searching this was my introduction to the remarkable Machimus polyphemi, the poorly known and beguiling gopher tortoise burrow robber fly.
Robber Flies (Family Asilidae) are especially abundant and diverse in gopher tortoise country, that is the arid, open-canopied sandhills of the southeastern Coastal Plain. A number of robber flies are characteristic of and/or highly specialized for life in these arid habitats. Over the last few years, their remarkable beauty and interesting habits have inspired me. A diverse group, there are around 1,000 species in North America, and at least 150 “robbers” native to the southeastern U.S.
Robber flies are recognized by their large, well-separated compound eyes, one pair of wings (like all flies, not two pairs as in bees and wasps) and a prominent tuft of bristly hairs (called a mystax) that covers and protects their beak-like proboscis. The tuft of hairs on the thorax is termed the pile.
Robber flies are predatory, and most are sit-and-wait ambush hunters that catch prey on the wing, which is unusual among insects. A hungry robber fly will perch on a fallen log, branch, or the tip of a leaf located in a sunny clearing (offering good visibility). When prey passes near, the robber darts from the perch, attacks the prey in flight, and then carries it to a nearby feeding station. Some species have dedicated perches – you might flush the fly, but wait a minute, it will return.
Immediately after capture, prey impaled on an impressively stout beak is injected with potent nerve toxins and enzymes that serve to paralyze (within 30 seconds to 1 minute!) and liquefy the food item. Robbers are not particularly finicky at the supper table, sucking the juices of spiders and a wide breadth of other insects including beetles, dragonflies, bees, wasps, and other robbers. Although larger species are capable of biting us if handled carelessly, robber flies never bite or attack humans. I have handled hundreds and think I got nipped once.
Let’s take a closer look at some species you may encounter at gopher tortoise sites.
Currently, I have a serious field crush on Proctacanthus heros. These hump-backed assassins, characteristic of open pinelands (especially sandhills) reach 45 mm, making them the largest robber fly in the eastern United States. Reported prey includes big stuff − bumblebees, large horse flies, dung beetles, burying beetles, and cicadas. The Florida panther of asilids, P. heros is not in large numbers even where common (congratulate yourself if you find more than one on the same walk). To look for these surprisingly unwary insects, walk near snags, stumps, large pine logs where they perch, cryptically, their reddish-brown bodies blending well with pine bark.
Also at home on those coarse and deep sugar sands is the sandhill specialist Stichopogon abdominalis. Like ghostly tiger beetles, they jet and bounce, disappear and reappear, ahead of us as we walk sand roads.
There are a number of robber flies that are remarkably convincing mimics of bumblebees, as well as major predators of hymenopterans. The enormous and wooly Mallophora bomboides (Note: this one has received enough attention that it has an established common name, the Florida bee killer) has been found in sandhills. An even larger western congener, the Bezelbub Bee-killer (M. leschenaulti), has been observed pouncing on ruby-throated hummingbirds as they rested at feeders. The historic robber fly literature includes some fascinating accounts of economic losses (during the late 1800s into the mid-1900s) to beekeepers courtesy of bee killers (and other large robber species). Like the splendid chain kingsnakes and fat yellow diamondbacks I find shadowing rat-heavy chicken houses in rural Georgia, the bee killers and company would simply perch near apiaries and pick off the relatively slow-flying honey bees from dawn to dusk.
The bumblebee mimics include representatives of two different subfamilies. They commonly prey on bumblebees; their wings create an audible buzzing in flight, reminiscent of a bumblebee. Summer is the time for a group of wonderfully strange robbers known as “hanging thieves” (genus Diogmites). These robbers fly painfully slow, drone-like, with legs outstretched as if parachuting. They perch hanging from only one or two legs when feeding. Robber flies across the board mate awkwardly, their abdomens connected and genitalia interlocked, but each fly facing away from each other. They can in fact fly away while doing so!
Members of the subfamily Leptogastrinae (e.g., Tipulogaster glabrata) are genuinely bizarre. These small, gracile and goggle-eyed boogers hover, then pluck prey from vegetation, or glean spiders from their webs. These clandestine flies can’t be found by conventional survey methods – to document them one needs to set Malaise traps, or, as I have done in our southern Georgia sandhills, sweep vegetation.
Machimus polyphemi (gopher tortoise burrow robber fly) is indeed an odd egg among asilids. An obligate commensal of gopher tortoise burrows, the species wasn’t described until 1991. It’s most unusual for a robber fly to live in, and depend on, an animal burrow, making M. polyphemi (and two western species in the same genus that inhabit the burrows of badgers) part of an ancient and highly specialized group.
In three days of field work, Giff Beaton, Delta pilot slash naturalist-entomologist extraordinaire, and myself doubled (from 5 to 10) the number of tortoise burrow robber sites known for Georgia. An insect that had taken on mythological properties was suddenly easy to find. The majority of active burrows we sampled housed M. polyphemi, but, as has been reported in the literature by the entomologists who described the species (see Bullington and Weck, 1991), we never found more than one of the golden flies per burrow. Just how do Mom and Dad meet? And, if an individual living in a prey-bountiful burrow expires, will another take its place? It is remarkable watching these flies dart, purposefully, straight back into their resident tortoise burrow only a few seconds after being flushed.
How did we do it? Serendipity. We discovered that our tortoise burrow dookie scooper (i.e., the macho trucker mug and accompanying six-inch-long free-hanging ends of the zip ties that secure macho to mop handle), when gently/repeatedly rubbed over the sand and hair-like rootlets of the burrow ceiling, served to flush the Machimus. From May-July they perch (0.5 – 1 m inside the burrows, occasionally on the carapaces of tortoises), the little barracudas-in-dipteran-suits regularly launching sallies for small insect prey−including another obligate commensal, the aforementioned anthomyiid, Eutrichota gopheri.
A wealth of information and many hundreds of awesome photographs of the robber fly fauna of the southeastern U.S. may be found on Giff Beaton’s webpage: http://www.giffbeaton.com
Bullington, S.W., and A.F. Beck. 1991. A new species of Machimus Loew (Diptera: Asilidae) from burrows of Gopherus polyphemus (Testudines: Testudinidae). Annals of the Entomological Society of America 84:590−595.