Asilidae : Dasypogoninae

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 very large numbers.

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 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. Note on this hanging individual (this is still angustipennis, being used to illustrate all these things) the little knob (the pronotum) midway between the main thorax and the head. The hairs standing up from it, seen against the pale rock, are yellow-gold in color. That will help to distinguish this from the next species.

Diogmites basalis

This individual has the same thorax with its non-contrastive marking as angustipennis, but the hairs coming out of the pronotum are black. In addition the wings on this individual are black, not clear as in angustipennis. There is a population of these in the woods across the street from my house (I live in Jonesboro, Craighead County, alongside the fattest part of the Ridge), and their identity is still being worked on. For now they are considered to be Diogmites basalis, which they most closely resemble, but they may be a new species.

UPDATE MARCH 3, 2008: I collected a series of these puzzling robbers and brought them to Jeff Barnes at the Arthropod Museum at the University of Arkansas. He has dissected them and determined that they are in fact merely a light-colored form of Diogmites platypterus (see below for that species), and so D. basalis has been removed from the list.

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 open woodland, landing on scattered grass stalks.

Diogmites missouriensis

This is Diogmites missouriensis, an aggressive species common in grassy meadows and also the poison ivy beds on the edges of closed-canopy woods. In this species, the contrastive pattern has the black mid-dorsal stripe continuing on down to the front of the thorax. Additionally, the stripe is divided in the middle.

The divided mid-dorsal stripe is obvious in this picture of missouriensis.

Diogmites Texanus

UPDATE MARCH 3, 2008: Here is a robber fly I think I have been overlooking because it is so much like D. missouriensis. Note the central black stripe going clear to the front of the thorax which is divided as on missouriensis (though the division is very narrow). What I finally noticed on it this past summer is that the abdomen was quite heavily marked with black, in the first few segments the black going clear across the dorsal surface of the segments. That is not how the missouriensis abdomen is marked, so I collected this one to examine closely. It is only when I looked at it under the microscope I noticed the wings were brownish fumose, the mark of Diogmites texanus, a completely unexpected species. I will try to see more of them this coming season, to see if the wing color can be seen in the field. Probably the heavily marked abdomen is the best field mark. This was on the edge of a weedy, grassy field in the second week of August.

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.

Diogmites platypterus

The final Diogmites species on Crowley's Ridge, Diogmites platypterus, is one of the commonest, abundant in gardens, weedy fields, and poison ivy beds in woodland, and is the most untypical, with its brown body, unmarked black abdomen, and black wings. It's another species with black hairs on the pronotum, one of many features it shares in common with what I have earlier called Diogmites basalis. 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, of all subfamilies and genera, 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.

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.