The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Dasypogoninae

Species of Crowley's Ridge

The next subfamily is the Dasypogoninae. These are much more typical Robber Flies than the preceding subfamily, and, with one exception, are in the 15 to 30 mm size range. The technical feature for recognizing this group is the thick claw-like spine at the end of the front tibiae (the second long joint of the leg). In this picture you can see it outlined against a grass blade at the end of the right fore-tibia. I mention this for interest; it is nothing you can see in the field to help with identification. This individual is in the Genus Diogmites, a very speciose genus generally, with seven species just on Crowley's Ridge. Five of the species are yellow like this one, with a long body and reddish legs, and the different species must be separated mainly on the basis of markings on the thorax.

Diogmites angustipennis

I will begin with the same individual, turned sideways so you can see it better. This is Diogmites angustipennis, showing the typical coloring and gangling shape of the genus as a whole. This particular individual has little or no marking on the thorax. Some individual angustipennis have a bit more, but the markings they have do not contrast very much with the rest of the thorax. D. angustipennis is found throughout the summer in fields of tall grass, or in sandy areas of scattered grass and other low vegetation, often in such large numbers they exhaust their food supply and end up preying largely on each other.

Here is a more strongly marked, but still typical, angustipennis. Individuals regularly shift from red to green eye color, perhaps due to light angle. This particular individual was photographed right where the St. Francis River enters the Mississippi River. What is interesting about it is, it is about half the normal size. I have observed that all angustipennis I see that are actually on the shore of the Mississippi River are this same miniature size. The Genus Diogmites in general seems to be evolving rapidly, so I wonder if this is perhaps an incipient new species in this special habitat.

Diogmites mate tail to tail, as do most of the more "primitive" Robber Fly species (many of the more "modern" species mate facing in the same direction, with the male above the female—generally holding her by the eyeballs).

All species of Diogmites (this is still angustipennis, being used to illustrate all these things) have the interesting habit of catching their prey, then flying under the shade of nearby vegetation and hanging by one or two forelegs while they eat their meal.

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Diogmites misellus

Here is Diogmites misellus. It has a highly contrastive pattern on the thorax, with a black stripe mid-dorsally, and shorter black stripes on either side. Note that the middle black stripe stops near the high point of the thorax, then turns red as it continues down to the front. Note also the strong marking on the side of the abdomen.

D. misellus is a small, slight species found in openings in woodland, landing on scattered grass stalks.

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Diogmites neoternatus

Here is an often large, very aggressive fly, Diogmites neoternatus. I find them in gardens and woodland edges, or openings in woods. These, like other Diogmites, love to feed on bees and wasps, but they also like to feed on other robber flies, especially other Diogmites. On neoternatus, the central stripe ends before it curves down to the front of the thorax. The front of the thorax sometimes turns red, somewhat like misellus, but—for a better recognition mark—notice on this species, unlike most other Diogmites species, there is no marking on the sides of the abdomen.

The absence of marking on the abdomen can be seen more clearly here. This neoternatus has killed and is eating a wasp that probably exceeds it in bulk.

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Diogmites missouriensis

Diogmites missouriensis has the central black stripe on the thorax continuing clear to the front of the thorax, and it is divided down the middle by a very narrow stripe which can just be seen in this picture (blow the picture up and it will be more obvious). Now note an absence of black marking on the dorsal surface of the abdomen, though there are some black marks on the sides. Finally, and most importantly, note the fairly transparent wings. The last two characters may help you separate this species from the next, very similar, species.

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Diogmites texanus

Diogmites texanus is common in grassy fields and in the poison ivy beds in woodland at the very same time that D. missouriensis is out in the same habitats. Like missouriensis, texanus has a black central stripe clear to the front of the thorax which also is narrowly divided down the center. But it also tends to have much blacker markings across the dorsum of the abdomen; if your eye is quick you may be able to notice what looks like black banding on the abdomen when this species takes off. However the only certain way to separate them is by the fact that missouriensis has relatively clear wings, whereas texanus has black or brown smoky, or fumose, wings.

Unfortunately the fumose wings on D. texanus cannot be seen when it is in its normal resting position, with the wings folded over its back, and so the two species are essentially indistinguishable in the field.

A lucky camera shot caught this individual just taking off, revealing the fumose wings. But this was too fast for the eye to have been able to see. When I want to be absolutely certain of the identification, I sometimes will net an individual and put it in a clear plastic vial, where, in its efforts to escape, it will often open its wings so that the smokiness is clearly visible.

These voracious creatures attack bees, wasps, beetles, other robber flies, and sometimes surprisingly large prey. Note how casually it supports its weight, and the dragonfly's weight, with the grip of one foot.

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Diogmites platypterus

Diogmites platypterus is the commonest Diogmites species on Crowley's Ridge, abundant in gardens, weedy fields, and poison ivy beds in woodland, and is the most untypical, with its brown body, unmarked black abdomen, and broad black wings. It mimics a Polistes wasp. When it flies from one place to another, it flies slowly, like a Polistes, and even lifts up its legs, as a Victorian lady would lift her skirts stepping through mud, and flies with the hind legs trailing out behind and curved upward, and this is exactly how a Polistes wasp flies. The advantage of mimicking a wasp is that a bird or lizard will hesitate to pick you up. The ironic downside is that D. platypterus is often caught by other, wasp-hunting, robber flies!

Here is Diogmites platypterus ovipositing in the ground, where the hatching larvae apparently find their way into the tunnels of beetle larvae, which the robber fly larvae either predate or parasitize. Most Robber Flies lay their eggs in the ground in this stereotypical way: They can be seen walking along the ground with their abdomens hanging down limply, but apparently sensitively feeling the ground, looking for a spot of soft earth, such as earth disturbed by ant diggings, which would be easy to dig in. When earth of the right consistency is found, the tip of the abdomen begins to sink into it. Though no movement is noted in the abdomen, little digging tools at the tip of the abdomen are working, and loose earth begins to appear in a circle around the abdomen. After a few minutes spent laying the eggs, the abdomen is withdrawn, and with the muscular dexterity (and intelligence) of a finger, the abdomen reaches around gathering in all the loose earth that has been dug up, and fills in the hole, and spends a considerable amount of time carefully tamping it down until no sign of the dig remains.

When the robber fly has completed its immature stages underground, the spiky and rather active puparium forces its way half-way out of the surface, for the adult to emerge.

This D. platypterus, which has just emerged from that puparium, has not yet obtained its mature coloring.

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Diogmites discolor

The much scarcer Diogmites discolor, with its dark coloring and black abdomen, is so similar to D. platypterus that it could easily be overlooked. I have found it in open or shrubby pine woodland, and, perhaps this is a coincidence, I have always found it on or adjacent to very steep hillsides. So if you see a dark Diogmites in this habitat, which would be uncharacteristic for D. platypterus, squat down as low as you can to see the sides of the abdomen. D. discolor has shiny very obvious silver triangles on each abdominal segment.

You can sometimes see the males of D. discolor fly slowly and persistently about two feet above the ground. You can follow them at a run, but, frustratingly, they never seem to land. I assume they are prospecting for females. They fly so slowly you can easily identify them by their shiny black abdomen with the silver marks on the sides like the lighted windows of an airliner. They look exactly like this specimen picture.

The genders of D. discolor are dimorphic. Instead of black, the female has a dark brown abdomen, but with the same silver marks along the sides.

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Nicocles pictus

One more species in the subfamily Dasypogoninae occurs on Crowley's Ridge. It is nothing like the Diogmites, and in fact is rather different from all the other species of robbers on the Ridge. It is Nicocles pictus. It is anomalous in the first place, because it emerges in the third week in February and flies until the first week of March, nearly a month before any other robbers come out on Crowley's Ridge. Before any leaves have come out it can be seen sitting at the ends of the bare twigs on tall bushes and sapling trees in early second-growth woods. It is also nearly unique among robber flies in having a second brood, emerging again at the end of October and the beginning of November, making it the latest robber fly to come out. At this time the leaves are also beginning to get thin, so there are once more abundant bare twigs for perching.

This specimen picture will show another anomaly. Quite unusual in robber flies, Nicocles pictus has patterned wings, black at the base, a black band along the middle, and a band at the tip. It is a primitive pattern that shows up in flies of many other families. Note also in this picture another curious feature: From most angles, the dorsal surface of the abdomen appears uniformly black. But from just the right angle, the last two segments shine up like reflective mirrors. It is only the males who have this feature.

A male sits at the tip of a twig at this 45 degree angle, the abdomen held straight and concealed under the wings. If your approach disturbs him, he will turn so his back is towards you, and with his brown coloring he will disappear against the twig.

But if he is not disturbed, he will continuously bend his abdomen down, then straighten it, like someone working a stiff finger, and at a certain point of bending down, it will flash its mirrors, presumably to catch the attention of any females in the area.

It occurs to me that, by being active at times of year when few leaves are present in the woods, he maximizes the distance from which his mirrors can be seen.

The female N. pictus has a bit of silver on the sides of the abdomen, but does not have the bright mirrors on the dorsal surface like the male. On this picture the banded wings can just be made out.

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