The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Stenopogoninae

Species of Crowley's Ridge

The Stenopogoninae are a varied group of Robber Flies often striking in appearance, always interesting in behavior. Many are superlative wasp mimics. There are five species on Crowley's Ridge, each in its own genus.

Heteropogon macerinus

This is Heteropogon macerinus. It is an autumn-flying robber, appearing in the third week of August. It is a smallish (but not tiny) fly that sits on twig ends at woodland edges at about eye level. The hunch-backed appearance, red legs, and long wings extending beyond the tip of the abdomen, are distinctive. (See below: The somewhat similar Ommatius have wings shorter than the abdomen. Also, see above: A late surviving Heteropogon may overlap with the second brood of the smaller, but similar appearing and behaving Nicocles pictus.))

If you are lucky enough to see the courtship, that will put the identification out of doubt. This rather poor picture is from a Texas species of Heteropogon, but 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.

The next step is for the female H. macerinus to disappear down a shady woodland path, where she finds a bare spot of soft ground to oviposit in.

Here she is using the tip of her abdomen to cover the hole and tamp it down.

[back to the top]

Holopogon phaeonotus

The next species of Stenopogoninae is Holopogon phaeonotus. This is a very tiny twig sitter. You can separate it, through your binoculars, from Cerotainia (see above) because Holopogon does not have noticeably long antennae. Against a contrasty background you might be able to make out that this species is very hairy, and the hair grows longer and longer towards the back of the thorax.

The female does not have the alternating bands of golden hair hanging down from the abdomen that the male often has.

A surprisingly good identification mark is that, from various angles of light, the wings shine up bronzy-purple.

On this mating couple (the male has the golden hairs under the abdomen) you can see a key feature to identify phaeonotus from other species of Holopogon: The hind tibiae get broader and broader, until at their apical end they are broader than the hind femora. This character is probably impossible to see through binoculars.

[back to the top]

Ceraturgus elizabethae

The next species is Ceraturgus elizabethae, an excellent wasp mimic. Many robber flies mimic wasps, but the imitation breaks down because wasps tend to have long antennae, and robber flies short antennae. Ceraturgus has overcome this by adding another joint to its antennae, so if you are able to recognize this species as a robber fly (and not a wasp), the long antennae will tell you it is Ceraturgus. It is found both in open meadows, and on the poison ivy leaves of closed canopy woods. When it catches its food it often hangs by one or two legs under vegetation to eat it, just like a Diogmites (see above).

[back to the top]

Echthodopa formosa

The fourth species of Stenopogoninae is Echthodopa formosa. It is a smallish slender black fly with orange tibiae that show up from as far away as you can see the fly. It vies with Laphria flavicollis to be the first fly out in spring on the poison ivy leaves. Often they appear on the same day sometime in the second week of April. This female has a white mystax, and whitish or very pale body hairs.

The male is distinctly oranger. Also, the base of his wings, looking down on his back, is noticeably silvery in color.

[back to the top]

Prolepsis tristis

The fifth species is the very interesting Prolepsis tristis. So far I have only found them at the southern tip of Crowley's Ridge, mainly at the mouth of the St. Francis River (which more or less parallels the east edge of Crowley's Ridge from top to bottom), right where it goes into the Mississippi. It is a species of the first high vegetation above a sandy river beach. That distinctive habitat will help in the identification. This is one form of the variable male. He is (like Orthogonis before him) a very good mimic of a Pompilid wasp. In this particular form he has a red tip to his abdomen, and when he flies slowly and persistently, with his long hind legs dangling below him, he looks very much like a wasp indeed.

In this other form, he is also very waspy with an all shiny black body, except a few white bands at the base of his abdomen. Note other features of the disguise: rather long antennae for a robber fly, to appear more like a hymenopteran, and observe how the mystax, combed down over the beak, almost looks like the biting mouth-parts of a wasp.

The female is a real surprise, not even looking like the same species. With her red legs and body, and the complicated red yellow and black banding of her abdomen, she perfectly resembles a Polistes wasp. Wasp expert Howard Ensign Evans has named this "dual mimicry" where the male and the female have different models.

If you don't believe they are the same species, here are the two mating.

[back to the top]