The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Ommatiinae

Species of Crowley's Ridge

The final subfamily to occur on Crowley's Ridge is the Ommatiinae. These somewhat mysterious robbers sit silently in the shade. They feature a round thorax, black and yellow legs, and strangest of all, feathered antennae (view this picture at fullest magnification to see the feathers). Many of the species appear identical to one another. Very close attention must be paid to the marking on the hind femora. I have found two species, and strongly suspect there is a third species, but I have not been able to confirm it.

Ommatius ouachitensis

In the beginning of June when you are walking along a shady woodland trail you will begin noticing a smallish round-thoraxed fly sitting quietly along a branch or twig in the shadows. With a closer look you will see it has black and yellow legs. But it is only after you have looked at your photograph of it against a contrasty background that you will see the most curious feature of this fly. It has feathered antennae. I have noticed that this robber is slightly crepuscular, so I wonder if the feathering has to do with some special senses needed in the failing light of dusk, when I have seen them gather in large numbers out in the open, perhaps for mating. This fly, in the subfamily Ommatiinae, is Ommatius ouachitensis. Note on this fly (and this is easy to see with binoculars, since these robbers tamely allow close approach): the black on the hind femur takes up at least one third of the length.

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Ommatius gemma

Along about the third week of June, another species of Ommatius shows up, also with feathered antennae: Ommatius gemma. It is noticeably smaller, and if you look at the hind femora, the black takes up less than one third of the length. It is usually less common than O. ouachitensis.

There is an infallible mark to identify O. gemma. It's probably impossible to see through binoculars, but a shot from just the right angle might get it in a photograph: On the scutellum (the little bump on the back just behind the thorax), O. gemma, and no other Ommatius species, has two stiff bristles standing up. You will see it here at the fullest magnification.

To make it easy: Here is O. ouachitensis, showing percent of black on the hind femora, and lack of twin bristles on the scutellum. Now compare with the next picture.

Here is O. gemma, showing reduced black on hind femora, and twin bristles on scutellum. And remember, O. gemma is usually noticeably smaller than O. ouachitensis.

To date, September, 2009, these 68 are all the species I know of that have been recorded on or near to Crowley's Ridge.

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