The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Asilinae

Species of Crowley's Ridge

The subfamily Asilinae is a heterogeneous group of, for the most part, smallish rather similar appearing Robber Flies. For many of them, leg color is the best clue to identity.

Asilus sericeus

The subfamily begins, however, with the largeish and very striking Asilus sericeus, the namesake genus of the Robber Flies as a whole (Family: Asilidae). There are more members of the genus Asilus in the Old World, but this is the only New World representative. It is a handsome golden fly that might, at a glance, be confused with Diogmites, except that it sits flatter to the ground, and — the best mark, perhaps — has broad black wings with yellow veins. It is a garden and woodland edge species, that feeds entirely on moths and butterflies. Here it has caught a Little Wood Satyr.

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Dicropaltum rubicundus

If you are walking through a dry field of grass and low weeds you might pick out a small, frail appearing robber fly down in the vegetation which features dark brown eyes, a long brown-banded abdomen, absurdly short wings, and rather stout legs. This will be Dicropaltum rubicundus.

Probably you won't be able to see this through binoculars, but the legs are covered with short pale hairs, and the genitalia are noticeably reddish (hence the name, rubicundus).

If you see D. rubicundus from the back, a very noticeable mark is the broad dark-brown stripe down the center of the thorax.

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Neoitamus flavofemoratus

This is Neoitamus flavofemoratus. It comes out in early spring more or less alongside Laphria flavicollis and Echthodopa formosa under the canopy of the woods in the poison ivy beds. However, instead of landing on the leaves, it lands on twigs at about eye level along the dirt path you are taking through the woods. It's not a tiny twig sitter, but it is rather small. Note that the femora on the middle and hind legs are black, but the front femora are largely yellow, as are all six tibiae. The yellow front femora, if you can make them out through your binoculars, will identify this species. Also note, but this is probably impossible to see in the field: the hairs behind the eyes up at the top of the head, against the light green of the leaf, can be seen to curve forward, a sure sign of Genus Neoitamus.

The female Neoitamus has genitalia composed of five segments, and the front femora are yellow. Note her white mystax; the male's is black.

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Philonicus rufipennis

Here is Philonicus rufipennis. You usually find it along the edge of a water course. It is somewhat variable, and can look confusingly like other species. It is small to medium sized, about 15-18 mm, and the best character is the reddish legs, with sometimes but not always a bit of black along the sides. Usually also there is a broad stripe of brown on the top of the thorax. This is the male with his forceps-like equipment at the tip of his tail.

Here is the female P. rufipennis with her short ovipositor. Note, again, the orange-red legs, and the dark mid-dorsal stripe. Note also the wings coming nearly to the tip of the abdomen, longer than in some similar species.

Here is a pair of P. rufipennis together. The red legs continue to be the best character. We are coming to a group of very similar appearing smallish robbers in which leg color is often critical for making identifications.

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Machimus Erythocnemius

On this rather similar robber, the wings are one segment shorter than on the previous species. Also note that the femora are black with a pre-apical gold ring.

That individual still can't be identified until you see it from this angle (and this is not easy, even through close-focusing binoculars). Note the front femur, which is outlined against the green leaf beyond. You can see a white bristle and two or three black bristles hanging down from it. The gold pre-apical rings and these bristles identify this as Machimus erythocnemius. You find this smallish species in early summer in the tall grass of remnant prairies, but also, confusingly, in the damp shady overflow areas of wet woodland.

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Machimus snowii

This nearly identical robber is Machimus snowii. It has the shortish wings, the preapical gold ring on the femora, but look again at the front femur: instead of three or four spines, it has a thick curtain of long white hairs. Again difficult, but not impossible, to make out through binoculars. This species is also a denizen of shady overflow areas of bottomland woods, but usually is not seen much before August, and is a fall species. However there are occasional sightings in June, which may be partly confusion with M. erythocnemius. I will have to work this out more carefully.

The flash usually picks up a lot of gold color in M. snowii. In this complicated picture the female has caught a moth and the male has seen this as a good time to mate. Note the eyeball hold.

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Machimus virginicus

This is another Machimus, Machimus virginicus. It is similarly shaped to the last two, but is rather black in color. It occurs in early summer under the canopy on the poison ivy beds next to the various Laphrias, Echthodopa, and Neoitamus. The critical field mark is to note the femora are completely black, but now the base of the tibiae have a gold or anyway orange ring.

M. virginicus (not looking quite so black out in the sunshine) is a very common robber in its habitat, and there is a period in early summer when you cannot go out without seeing one or more pairs mating. Note again the gold ring at the base of the tibiae. The inside of the hind tibiae and tarsi are often yellow.

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Machimus maneei

This is a fairly new discovery, Machimus maneei. I found it in October, on a jumble of fallen pine logs, the product of a fire several years earlier. It is quite black, with a strongly banded abdomen, and legs which to the eye are entirely black, but under the microscope, or blown up in this picture, can be seen to be slightly reddish at the base of the tibiae.

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Neomochtherus auricomus

And this last species in the Asilinae subfamily, a Machimus relative, appears in late summer in generally open upland woods. Rather than landing on or near the ground like most Machimus, it is usually seen clinging to the upright stalk of a low plant, or perching along the bare limb of a tree at about eye level. Since I have only found females, it is difficult to be certain of the identification. But it seems to me to key out most closely to Neomochtherus auricomus.

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