The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Leptogastrinae

Species of Crowley's Ridge

The Robber Flies (Family: Asilidae) are represented by ten subfamilies on Crowley's Ridge. I mean to take them up in phylogenetic order, beginning with the most "primitive," meaning the least changed from the original robber fly stock. These, the Leptogastrinae (meaning "slender abdomens") are quite unlike the more "modern" robbers. Robber Fly species go from about 5 mm long to about 50 mm. This particular group is on the small side of the scale, 7-15 mm long and extremely slight, so that they are about like mosquitoes in bulk. Their head and thorax are crammed up to one end, with enormously long hind legs that dangle down in flight, perhaps to mimic small wasps, and probably to serve as a counter balance to the long slender abdomen that takes up most of their length. It took Cheryl and me about three years to even learn how to see these thread-like things, flying slowly and persistently deep down in the shadows and twigs of brushy thickets. But once you learn to recognize the slow deliberate flight, and the special way they land, head against a bare twig, body often at right angles, to look like another twig coming off the main twig, then you have a search image, and it is possible to begin finding them.



Leptogaster atridorsalis

We have found five species of Leptogastrinae on Crowley's Ridge, and I expect we will eventually find one or two more species. Even with close-focusing binoculars most of them probably cannot be identified in the field. The best thing to do is to try to photograph them with a digital camera and study them in the viewing screen (and even that is not very easy to do). As I get good live pictures of them, I will replace those that so far I only have specimen pictures of. This first is Leptogaster atridorsalis. Note the hind femora: For half their distance they are slender, then they suddenly bulge out. The color on the bulge is black with a yellow band in the center. The hind tibiae are white on the basal half, black on the apical half. This will separate it from the other four species we have found on the Ridge. I have seen this species in the second half of June.



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Leptogaster brevicornis

This is Leptogaster brevicornis. It is the commonest Leptogaster on the Ridge, and when you find one, you will often find several more nearby. The strongly banded hind legs can be seen through your binoculars, and will identify this species. They haunt brushy twiggy dark places on the edges of woods, flying and walking over the abundant sheet webs of the spiders I believe they feed on. This one is pretending he is the end of that broken-off twig.


Let me revert to a specimen shot of brevicornis to show the recognition marks more easily. Note that the pattern on the hind femora is reverse from atridorsalis: The leg is yellow, with a brown band. But since another species has this pattern, you will also need to note the marking on the hind tibiae: light-dark-light-dark-light. (Caution: Another Arkansas species, Psilonyx annulatus, has this light-dark pattern on the hind tibiae, but so far I have never found that species on Crowley's Ridge. If one does show up, then all you will have to remember—as if you weren't confused enough already—is that Psilonyx has the hind femur pattern reversed, and is brown with a yellow band in the middle.) L. brevicornis flies throughout June.



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Leptogaster flavipes

Here is Leptogaster flavipes. It features a silvery thorax and mainly yellow hind legs, save for a couple of smudges (but no clear pattern) of brown. This species inhabits the open understory of pine/oak woodlands.



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Leptogaster virgata

Here is Leptogaster virgata. The hind femora are, as with brevicornis, yellow with a brown band (though through binoculars you tend to read it as a yellow band with dark on either side), but on this species the hind tibiae are mainly dark. But the best recognition mark is on the thorax: The dorsal half of the thorax is silvery pollinose (under a microscope you see the silvery is made up of dense tiny white hairs). But a stripe down the center of the thorax, and shorter stripes on either side of the mid-line, are shiny black, and look as though the white hairs have been worn off to expose the shining bare thorax beneath. If this feature can be seen through binoculars, it will positively identify the species. These are found in open woods In June and July.


For clarity, here is a specimen picture of L. virgata.



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Tipulogaster glabrata

Finally, here is Tipulogaster glabrata. It is large for a member of this subfamily, almost 15 mm. Note the shiny red-brown dorsal surface of the thorax, almost as if it were painted with lacquer. The hind femora are brown with a rather narrow black band. This species, at least, is readily identifiable through binoculars.



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