The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Stichopogoninae

Species of Crowley's Ridge

The Stichopogoninae are a group of small to tiny robbers that occur, often in large numbers, on open sandy or rocky places, such as river beaches or abandoned quarries. So far we have found two members of this subfamily on Crowley's Ridge.

Stichopogon trifasciatus

Stichopogon trifasciatus has the longest flight period of any Arkansas robber, coming out in the second week of May and continuing on through the first week of November, which is nice because it is a particularly charming fly. It often abounds in open sandy or rocky places, often but not always near water. It is a smallish fly, but easily identifiable because you can plainly see it has two black abdominal segments, then a blue-gray segment in the middle, followed by three more black segments. In good light the legs appear blueish. In its sandy or rocky haunts (a former quarry in Jonesboro, now part of The Nature Center, or in the riprap beside concrete boat landings or bridges) it is often accompanied by tiger beetles, and the two are about the same size and look rather similar flying up ahead of you as you walk.

S. trifasciatus is often quite sociable.

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Townsendia nigra

A rather surprising new find is Townsendia nigra, at 4-5 mm considered one of the smallest robber flies, and usually found along the Gulf Coast, but now we have found a colony along the St. Francis River in the extreme northeast corner of Arkansas on the border with Missouri. To give an idea of its size, here it is (on the left) next to a quite ordinary-sized fly.

The habitat is sandy riverfront with scattered grass and small scattered trees and bushes on the edge of a woods. The Townsendia nigra we have seen landed within an inch or two of the ground, but on a small stick or stalk or other such object that lifted them up just above the ground. So, a very tiny dot landing just above the sand on a twig or dry grass stalk might be this fly, but it is a kind of miracle to spot it, and even with close-focusing binoculars hard to tell if it is a robber fly. There are many wasps in this size category, but you can usually see their long antennae. Watch for the typical robber fly behavior of flying up after prey, then returning to the same perch.

As often with robber flies, the male is even smaller, in this case probably under 4 mm. It is landed on the stem of a fallen leaf.

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