The Robber Flies of Crowley's Ridge, Arkansas
An Illustrated Field Guide by Norman Lavers
When my friend Herschel Raney (see his much fuller website at https://www.hr-rna.com/RNA/index.htm) and I began studying Robber Flies in Arkansas about six years ago, all we knew is what was, and what wasn't, a Robber Fly. We acquired a Key to the Genera of North American robbers, and we had to catch each new fly we saw, key it to Genus, find a paper that discussed that Genus and contained a Key to the Species, and send for the paper, and key it out.
At the end of each season we donated all our specimens to the Arthropod Museum at the University of Arkansas, and the curator, Jeff Barnes, checked our ID's and corrected our mistakes. But little by little we used our birdwatching skills to find field marks for each species, so that we could identify it in the field without having to catch it. Now we can identify almost every species we see, unless it is something new. It is those field marks I will try to convey here.
All photographs here are by Norman Lavers or Cheryl Lavers. Rather than using idealized portraits, we have tried to select the images which looked most like the way species appeared in the field. Although our focus of study is a small area of northeastern Arkansas, in fact the majority of species here are found throughout Arkansas and adjacent areas of Oklahoma, Missouri, Tennessee, and Louisiana. We have now begun, as an appendix, "Other Arkansas Robber Flies," which ultimately will include every species of Robber Fly anyone is likely to see in the state as a whole, and most of those likely in the region.
So far as I know, this is the first attempt by anyone to create a guide to the identification of Robber Flies in the field. Since I am learning how to do this as I go, I would appreciate corrections for any mistakes I have made; hearing about additional field marks that I have missed; and especially I would like to know if this guide is useful in helping you to identify Robber Flies. What I really want is to make you aware of these fearless and fascinating predators that are all around you, though you may never have noticed them before.
What is Crowley's Ridge?
Crowley's Ridge is an anomalous feature rising out of the otherwise flat Mississippi Delta in northeast Arkansas. It is possibly a unique feature in the world, tilted up by an earthquake in the late Cretaceous, when the area was at the bottom of an inland sea. The 1811-1812 New Madrid earthquakes show this is still a region of puzzling seismic activity, despite being far from any plate boundaries. The area is considered a "failed rift" where the continent thought about dividing, then changed its mind.
Crowley's Ridge begins at the Missouri border in the northeast corner of Arkansas and runs south for 150 miles, bisecting parts of eight counties, before it meets the westward-trending Mississippi River. It is nowhere more than a few hundred feet high, or a few miles wide.
The soft and highly erodible loess soil has helped create a landscape of high ridge trails and steep-sided deep ravines.
The Ridge, an island in the Delta, long isolated from other highlands, and relatively undisturbed by agriculture and the periodic flooding of the bottom lands, has its own distinct botany, including many relict species, and this extends to some insect species as well. My wife Cheryl and I have been studying Robber Flies (Family: Asilidae) here since 2003 and so far have recorded 66 species. Another 2 species have been recorded in the literature. These flies are primary predators in their own ecosystems. I would like to present here an illustrated field guide to these fierce and rather charismatic creatures.
What is a Robber Fly?
Here is a typical Robber Fly, showing general features. Note the eyes set wide apart for good depth perception, with even a depression between the eyes to allow full sweeping vision. Also characteristic is the more or less hairy face (the "mystax"), which, it is theorized, protects the eyes during battles with large and dangerous prey. Now note the high arching thorax containing the powerful muscles which activate the wings, allowing it to fly up in an instant and snatch fast-flying prey out of the air. Robbers (unlike many similar appearing flies in other families) normally land with their wings folded together over the back of their extended abdomens, and this will help you pick them out. Note the long spiky muscular legs with which they capture and hold their prey (see especially the falcon-like talons at the ends of their legs). Hidden beneath the mystax (in this picture) and no doubt stabbed into the head of this hapless bee is a sharp beak backed by neurotoxins and digestive juices, which allow it to kill its prey (very often bees or wasps) quickly, and suck their juices out, like the tiny (often green-eyed) draculas that they are. No wonder entomologists are fond of them!
Robbers are in the Order Diptera, the true flies, characterized by having two wings (instead of the four wings typical of other orders). For engineering reasons I don't understand, two wings are much more efficient than four wings, and flies are the champion flyers of the insect world. Their hind wings have been reduced to little balancing organs (the halteres), which you can see on this fly as yellow knobs down below the wing bases.