Courting Robber Flies

It must be spring. Love is in the air, on twigs, on the ground....at least if you are watching insects. They are courting and mating everywhere. My last couple of outings afield on the plains here in eastern Colorado have given me the opportunity to witness some insect romance up close; and I remembered to use the video function on my camera, too.

Male Ablautus(?) robber fly with leafhopper prey

One particularly abundant insect in early spring is a small robber fly, family Asilidae, possibly in the genus Ablautus. Adults of this dipteran measure only about 6-7 millimeters, and they are sand-colored, so rather difficult to see. They prey on insects smaller than they are. The one in the image above has killed a leafhopper.

I discovered that the males are slightly smaller and more slender than females, with bright white hairs on their faces. Females have gold or ochre-colored hairs on their faces.

Female Ablautus(?) robber fly

Males are very energetic, and once engaged in courting a female, they are not easily frightened away. So, once I discerned there was a pair of the flies on the ground in front of me, I stopped and watched. This is the amazing result. The male alights near the stationary female, then flies and alights again, sometimes sidling into a different position. Eventually, he waves his front legs in front of her and rocks up and down. It is quite a display.

This next video is of a different pair of flies, perhaps a little farther along in the process, because it has an, uh...."happy ending." The male is the one with the smile on his face.

Many flies have elaborate courtship rituals, but asilids (robber flies) have some of the fanciest. I was lucky enough to also capture video of the aerial display of Heteropogon macerinus in Ohio a couple of years ago.

Robber fly "wedding photo"

I urge you to find your own examples of "bug love," and document them with your camera, even your smartphone. There are plenty of places to share the results online. Maybe your video will even go viral on YouTube. In any event, we stand to learn a good deal more about insect behavior through the efforts of curious citizen scientists.

Robber Fly: Heteropogon macerinus

Happy “Fly Day Friday.” Robber flies in the family Asilidae have to be among the most compelling of all insects, as evidenced by the many professional and amateur entomologists who study them. They are diverse in appearance and behavior, and yet there is still much to be learned about them.

This past August I had occasion to visit southern Ohio, and in several places I found specimens of the species Heteropogon macerinus. They are not terribly large, about 10 millimeters long or so, and fairly drab in appearance as you can see from the images here. They are fairly easy to spot, however, because they tend to frequent forest edges where they habitually perch on the tips of twigs at about eye level.

Robber flies like these remind one of flycatchers in the world of birds. They survey the world from their perch, cocking their head this way and that when an insect passes by. Should a flying bug seem to be a potential meal, the robber fly dashes out, grabs its victim, and returns to its perch to feed.

Robber flies have piercing-sucking mouthparts much like a mosquito, but shorter, stouter, and used to prey on other insects, not to feed on the blood of vertebrates. What is visible to us is the sheath that houses the stylets that do the actual piercing, so while it appears that a robber fly has a thick, knife-like beak, the killing and feeding machinery is virtually hair-like.

Asilids have an uncanny ability to find the weak spots in another insect’s armor, and quickly thrust their mouthparts into those areas. Some species go for the “neck,” stabbing through the thin and flexible exoskeleton that joins the head and thorax. Some are able to drive their stylets into the membranes between abdominal segments; or in the seams between various segments of the thorax as this H. macerinus has apparently done to a winged ant in the image below.

What really got my attention was a courting pair of Heteropogon macerinus that my wife and I spotted at the edge of a cemetery in Lynx, Ohio on August 25, 2013. The tarsi (“feet”) on the front legs of male Heteropogon species are at least slightly decorated with ornamental setae (“hairs”), and he uses these “bracelets” to full advantage. Norm Lavers, on his own robber fly website, describes their behavior most eloquently:

”If you are lucky enough to see the courtship, that will put the identification out of doubt….they all seem to do it the same way. The female makes herself visible by sitting on a twig usually rather high in the air, on the top of a sapling or tall bush. The male hovers before her, his hind legs out wide to the side, perhaps to show off the red coloring. The female responds by throwing up her hind legs (whether that means yes or no I don't know). If the male is allowed to get close enough, he begins stroking her eyeballs with his front claws while he is still hovering, a delicate operation, one presumes. If she likes him, she will eventually let him sit down beside her, where after a while one thing leads to another.”

Fortunately, another person in our party remarked that it would be wonderful to have a video of the aerial dance we were watching there in Ohio, and I remembered my camera has video capabilities. Here is the resulting….pixelage?

Is it any wonder why entomologists become enamored with asilids? The life history of the majority of species remains a mystery. The larval stage of those few that are known seem to be external parasites of beetle grubs, or the larvae of other insects. Your own observations could yield vital new information as to predator-prey relationships, and/or larval hosts.

I have yet to find any place in North America that does not have a pretty diverse assemblage of robber flies, so I challenge you to go looking, even in your own backyard. I will also bet you are likely to get addicted to these charismatic flies once you find them.

Sources: Hull, Frank M. 1962. Robber Flies of the World: The Genera of the Family Asilidae. Washington, DC: Smithsonian Institution Bulletin 224, Parts 1 and 2. 907 pp.
Lavers, Norman. 2007. “Asilidae: Stenopogoninae,” The Robber Flies of Crowley’s Ridge, Arkansas.
Preston-Mafham, Rod and Ken. 1993. The Encyclopedia of Land Invertebrate Behavior. Cambridge, MA: The MIT Press. 320 pp.
Raney, Herschel. 2009. “Heteropogon Page,” Random Acts of Nature.

Fly Day Friday: Comantella robber flies

Here along the Front Range in eastern Colorado, among the first flies of spring are unique little robber flies in the genus Comantella. When I first saw one I thought it looked like a march fly (Bibionidae), but behaved like an asilid.

Comantella are also among the last flies seen in the fall, as demonstrated by the above image taken on Novermber 29, 2011 in Colorado Springs. This unusual seasonal appearance of the adults (early and late), plus the presence of a slender spur on the tip of the front tibia (“shin” segment), helps to separate this genus from similar genera in the Asilidae. They are decidedly humpbacked when viewed in profile. Little appears to be known concerning their biology, though it is suspected that the adult flies overwinter in protected places. One specimen of the species C. fallei was observed preying on a small wolf spider in the genus Pardosa (Dennis, et al., 2012).

The taxonomy of Comantella is not very straightforward, either. At least one expert on asilids believes the genus is in bad need of revision. Current keys to species seem to revolve around the length of the terminal style as compared to the third antennal segment, and coloration of the hairs on the body.

These are small or medium-sized flies, 10-15 mm in length, and covered in long hair, even on their legs. The top of the thorax features a “mane” that can best be described as a Mohawk hairstyle.

There are only four currently-recognized species of Comantella. C. pacifica is found from British Columbia south to Nevada and Utah. C. cristata is known from Idaho and California. C. rotgeri occurs in Colorado, New Mexico, and Alberta. C. fallei ranges in California and Colorado.

I expect to see more of these flies any day now. Last year I found this mating pair on March 12, but the winter was very warm. In all instances I have found Comantella perched on the ground (rarely on vegetation), usually on bare soil, in vast open fields.

Sources: Cannings, Robert A. 2012. “A Checklist of the Robber Flies (Diptera: Asilidae) of British Columbia,” University of British Columbia.
Cole, Frank R. 1969. The Flies of Western North America. Berkeley: University of California Press. 693 pp.
Dennis, D. Steve, Robert J. Lavigne, and Jeanne G. Dennis. 2012. “Spiders (Araneae) as Prey of Robber Flies (Diptera: Asilidae),” J. Entomol. Res. Soc. 14(1): 65-76.
Dennis, D. Steve, Jeffrey K. Barnes, and Lloyd Knutson. 2008. “Pupal cases of Nearctic robber flies (Diptera: Asilidae),” Zootaxa 1868. Auckland, New Zealand: Magnolia Press. 98 pp.

Hull, Frank M. 1962. Robber Flies of the World. Washington, DC: Smithsonian Institution Bulletin 224. 907 pp. (2 vols.).
James, Maurice T. 1941. “The Robber Flies of Colorado (Diptera, Asilidae),” J. Kans. Entomol. Soc. 14(1): 27-53.
Lavigne, R.J., William F. Barr, and G.M. Stephens. “Checklist of the Asilidae of Idaho,” College of Idaho. 26 pp.
Neslon, C. Riley. 1987. “Robber Flies of Utah (Diptera: Asilidae),” Great Basin Nat. 47(1): 38-90.

Robber Flies

Robber flies are conspicuous denizens of the desert southwest, but are common nearly everywhere. You might be seeing them yourself but simply not recognizing what they are. No wonder. Robber flies, also known as “assassin flies,” often resemble wasps or bees more than they do flies. They certainly bear little resemblance to house flies. They don’t carry diseases or bite people, either. Instead, they are swift predators of other insects.

There are nearly one thousand species of the family Asilidae in North America north of Mexico, but that doesn’t mean they have all been discovered yet. I helped discover a new species of Laphria myself, in a park in Cincinnati, Ohio. Prior to my specimens, the species (still awaiting formal description) was known from only one other specimen collected in Pennsylvania.

Robber flies vary greatly in size (3-50 mm), shape, and color pattern. Some are robust bumble bee mimics. Others, like those in the subfamily Leptogastrinae, are slender and nearly invisible as they navigate through tall grasses.

There are some things to look for that are common to all robber flies, however. They have a deep, concave area between the eyes at the top of the head. This helps set them apart from similar flies like mydas flies (Mydidae) and dance flies (Empididae). Robber flies also often have a “bearded” face, with long hairs over their mouthparts. Even with the setae, the stout beak-like mouthparts are often visible.

Look for robber flies in a variety of habitats, from deserts to grasslands to forest openings with dappled sunlight. Robber flies like to perch on the ground, rocks, logs, tree trunks, or foliage where they have a great vantage point to scan the landscape and the sky above them.

Watch one on the tip of a twig or a leaf as it cocks its head toward insects passing overhead. The fly may leave its perch abruptly, but you should stay put. It may well return with a victim to dine on. Asilids are able to intercept flying insects in mid-air, much like a flycatcher bird does. Some species seem to have their food preferences, but most are generalists. Very large robber flies can kill insects as large as adult grasshoppers, or even dragonflies.

Members of the genus Diogmites are known as “hanging thieves” for their habit of swinging from the front or middle pair of legs while manipulating prey with the remaining two pairs, as the one above is doing with a skipperling it caught.

The bite of robber flies is administered with that beak, driven into joints in the exoskeleton of the prey insect. The “necks” of insects are especially vulnerable, and flying beetles are impaled while their elytra are open, exposing chinks in their armor. Paralytic compounds and digestive enzymes are likely injected during the bite as victims cease struggling almost immediately. The fly then withdraws the liquefied internal tissues of its meal.

As easily as adult robber flies are observed, the corresponding larval stage remains quite a mystery. Those species that have been reared are known to be external parasites of beetle grubs, or other insect larvae.

Before you set out into the field to find robber flies, it might help to become familiar with all the different genera you are likely to encounter. Remember that they often resemble insects other than flies. A good place to start learning is the image gallery at BugGuide.net. Click the “images” or “browse” tab near the top of the page to get more images. Another outstanding internet resource is the robber fly site built by Herschel Raney, an extraordinary nature photographer and self-taught expert on asilids. Three individuals are webmasters for a global robber fly website.

As the popularity of these amazing insects continues to grow, no doubt more resources will be created. For those who prefer printed to electronic references, my Kaufman Field Guide to Insects of North America includes two plates devoted to robber flies, with complementary text on the opposite pages.

Stop, listen (for a loud, droning buzz that stops abruptly) and look (carefully, for despite their size robber flies can be rather cryptic) for these winged wonders of the insect world. You will not be disappointed that you did.