A Case of Predator Mimicry in the Bee Fly Genus Epacmus? (Diptera: Bombyliidae)

In the course of photographing insects in Colorado Springs, Colorado, USA, at a nearby vacant expanse of prairie, soon to be a new housing development, I stumbled upon something interesting. Not until I got home and reviewed my images did I recognize something startling on an otherwise ordinary little bee fly in the genus Epacmus (assuming my identification is correct). Whether this has been documented before I do not know, but the phenomenon is well known in other insects.

Epacmus sp. bee fly. Nothing to see here....yet

Predator mimicry, or elusive mimicry, or aggressive mimicry, is when a prey species mimics one of its predators. This is probably a more widespread strategy than we currently recognize because we are not at the same scale as the predator and prey, and cannot easily interpret what constitutes such mimicry. We do not even enjoy the same perspective, so rarely experience the full effect.

It was not formally recognized, or at least not published, until 2006, that metalmark moths in the genus Brenthia have underside wing patterns that greatly resemble the face and legs of an oncoming jumping spider. They eye arrangement, chelicerae (jaws), and front two pairs of legs are all illustrated on the wings of the moth, when properly displayed by the insect. The moth would make an easy meal for the spider, but not if it presents the illusion that it is a spider. Evolutionary genius.

Spider mimicry is also exploited by other insects, including some fruit flies (family Tephritidae) and planthoppers (Fulguroidea). The bold patterns on the wings of some fruit flies greatly resemble the leg posture of some spiders. Some of the flies enhance the graphic imagery by moving their wings deliberately in a manner utterly convincing of a spider’s movements.

Whoah! Spider eyes and "mustache"

This brings me to the little bee fly I noticed on flowers in that prairie habitat. Viewed from above, Epacmus is an attractive, tapered, fuzzy insect about 7-10 millimeters in length, with delicate wings and something of a smiley-face pattern where the abdomen meets the thorax. Cute. Look at one directly from behind and you get an entirely different picture. Suddenly, you see the big black eyes of a jumping spider. Four black, polished, hemispherical bumps on the rear of the thorax are a perfect match for the eyes of a small salticid (jumping spider). This is reminiscent of raised features on the thorax of the Brazilian fruit fly Ceratitis alba (See Hill, David E., et al. 2019).

Habronattus sp. jumping spider stalking a Andrena sp. mining bee

Ok, so how often might the fly encounter a jumping spider? More frequently than you might imagine, even on flowers. Another photo I took of a mining bee revealed a jumping spider right on her tail. I hadn’t noticed the spider until I looked at the image later, at home. Habronattus jumping spiders are abundant in this prairie habitat, and hunt exactly where you expect to see bee flies: on the ground, vegetation close to the ground, and on flowers. The spiders are small and cryptic, easily overlooked.

Male Epacmus bee fly

What else are we missing in our observations of insects and their spider predators? Keep observing and you, too, may make a startling addition to our collective scientific knowledge.

Note: At least one world authority on bee flies (Bombyliidae) asserts that Epacmus may not be a valid genus, but a subset of the genus Aphoebantus instead.

Female Epacmus bee fly

Sources:Rota, Jadranka, and David L. Wagner. 2006. “Predator Mimicry: Metalmark Moths Mimic Their Jumping Spider Predators,” PlosOne.
Mather, Monica H. and Bernard D. Roitberg. 1987. “A Sheep in Wolf’s Clothing: Tephritid Flies Mimic Spider Predators,” Science 236: 308-310.
Hill, David E., A.P.C. Abhijith, and Joao P. Burini. 2019. “Do jumping spiders (Araneae: Salticidae) draw their own portraits?,”Peckhamia 179.1
Melander, Axel Leonard. 1950. “Aphoebantus and its Relatives Epacmus and Eucessia,” Annals Ent. Soc. Amer. 43(1): 1-45.
Hull, Frank M. 1973. Bee Flies of the World. Washington, DC: Smithsonian Institution Press. 687 pp.
Cole, Frank R. 1969. The Flies of Western North America. Berkeley: University of California Press. 693 pp.

Spider or fly?

Bee Flies

The title above is not a noun and a verb, but the common name for the family Bombyliidae. Bee flies get that name from their obvious resemblance to bees, but it is a diverse family and not all of them are covered in fuzzy hairs or iridescent scales. Their life histories are likewise variable, and for most species are unknown. At least 800 species occur in North America north of Mexico. More await naming and description by scientists.

Spring is a great season to see bee flies. Look for them in open areas on sunny days, or even sunny spots in wooded areas. Many species, like the “greater bee fly,” Bombylius major, shown above, hover so perfectly as to appear to be levitating. Male bee flies in particular exhibit this behavior, perhaps defending small territories or displaying for females. You might easily hear one of these flies before you see it.

Many bee flies have elongated mouthparts, causing some people to fear them as blood-suckers. The proboscis is for probing flowers for nectar, though, and the flies are totally harmless to people and pets. Bee flies are probably mostly “flower visitors” rather than pollinators.

The larval stages of bee flies are definitely not harmless to other insects, and this makes them beneficial allies in the war against pests. At least some members of the genus Geron, shown above on an aster in western Massachusetts, are parasites of the caterpillars and/or pupae of bagworm moths in the family Psychidae. Bombylius major, a widespread species illustrated near the top of this article, is somewhat less helpful. Its offspring are parasitic in the nests of solitary bees that nest in the soil. This and related species are known as “bombers” because the female will hover over the entrance to a bee’s burrow and lob eggs into it with a flick of her abdomen. The larvae that hatch then seek out the host bee’s larvae, becoming external parasites upon them.

Members of the genus Villa, like the unidentified specimen from Arizona pictured here, are villains only to caterpillars of butterflies and moths, again functioning as parasites in the flies’ larval stages.

Larvae of Hemipenthes are hyper-parasites. That is to say they are “parasites of parasites.” They live as parasites of the larvae of ichneumon wasps, tachinid flies and other insects that are themselves parasites of caterpillars of butterflies and moths. How bizarre is that? I photographed this Hemipenthes eumenes (below) a little over a week ago here in Arizona.

Try looking for bee flies yourself next time you are out and about. You might also surf the ‘net to get a feel for what they look like. Start by browsing the images BugGuide.net for a great overview of the North American fauna. See more stunning images from all over the world at this Flickr search. You’ll be amazed by the beauty of these delicate insects.