The Robber Flies of Crowley's Ridge, Arkansas

An Illustrated Field Guide by Norman Lavers

Asilidae : Apocleinae

Species of Crowley's Ridge

The final three subfamilies contain the most highly "evolved" or derived species, the most altered from the primitive state of Robber Flies. They are the equivalent of the Passerines among birds. The first of these subfamilies, the Apocleinae, contains the commonest, the most "typical," and some of the largest and most formidable, of the robbers.



Efferia (Nerax) aestuans

The first genus in this group, and one of the largest genera in the United States, is the Efferia. The genus is well marked: Most have black femora and red tibiae. The males have upturned genitalia, canted at an angle from the rest of the abdomen; females have long sword-like ovipositors. Efferid species also have in common being more or less average in size, 15 mm to 25-30 mm. There are no more dots among these more advanced robbers. Females can be difficult to identify, but males generally have differing numbers of abdominal segments covered by silky white hair, and this helps to separate them. Seven species occur on Crowley's Ridge. This first is Efferia (some authorities say Nerax) aestuans. Note first of all the largely black mystax, which separates E. aestuans from all other Efferids on the Ridge. But then notice that on this individual there are three-and-a-half segments on the abdomen that are white.


Now notice on this one there are four-and-a-bit segments white. For this reason these were thought to be two different species, but now it is known that these are two forms of the same species: E. aestuans. Just remember on this species both forms of male, and the females, have (1) a black mystax, (2) a curtain of long white hairs hanging down under the abdomen, and (3) wings extending clear to the end of the abdomen (the genitalia extending beyond). The black mystax, remember, is particularly helpful, no other Crowley's Ridge Efferids sharing it.


E. aestuans, as the various pictures will show, also seem to come in a brown, a gray, and a blond form. Just remember to check the mystax color, which is always the same.


On the female E. aestuans, in common with most female Efferids, there is a long sword-like ovipositor (note the female here still has the black mystax, long wings, and hanging white hairs of E. aestuans). This one is not looking for the sports page. She is feeding on some kind of plant hopper with voluminous wings.


This E. aestuans is feeling for a crevice in this stick to lay her eggs in. I have often seen them laying their eggs in dried flower heads. Species with long ovipositors do not oviposit in the ground but look for such places as these to inject their eggs into. My guess is that when the eggs hatch, the larvae drop to the ground, and set off looking for their prey.


From time to time, many kinds of Robber Flies will come over and land on you. Perhaps you make a nice high observation post. At any rate, if a Robber Fly lands on you, more often than not it will be this very common species, Efferia aestuans.



[back to the top]


Efferia (Albibarbefferia) albibarbus

Here is Efferia albibarbus (some authorities say Albibarbefferia albibarbus), found in open sandy areas such as river beaches. It has sort of dirty-white facial hair, and on the male two-and-a-half white segments at the tip of the abdomen. E. albibarbus land on the ground and with their sandy color disappear into it. The white segments of the abdomen are visible from a distance, but as you can see, when they think danger (or an annoying photographer) are nearby, they cover the white with their wings.


When they don't feel danger is present, they open their wings to display the white (whenever the male has some bright color the female lacks, one assumes it is meant to be used to signal the female). The male, seeing me coming, is actually in the act of covering it up again. There is, by the way, a good identification feature here, but generally difficult to see with binoculars: The first segment of the abdomen has a brown spot on each side of the dorsal surface—not all the way across, as in succeeding segments.


Females (this species has a rather short ovipositor) are scattered about, sitting flat on the sand. What I have seen is this: Suddenly a female flies up and flies directly over a displaying male, perhaps a meter above the ground. The male darts upward, like he is going to capture prey, but comes down grasping the female.


He lands above her, facing in the same direction, and uses the eyeball hold that no female can resist.



[back to the top]


Efferia (Pogoniefferia) nemoralis

Efferia nemoralis (some authorities say Pogoniefferia nemoralis) is a very handsome robber. They are not always as dark as this one. The mystax is yellow, the legs are contrasty black and red, the bulb is shiny black, and four abdominal segments are involved in the white hairs, the first two segments having long hairs swept down on either side. The wings are a bit shorter than on E. aestuans.


The female E. nemoralis has the yellow mystax, constrasty black and red legs and a long black ovipositor.



[back to the top]


Efferia (Pogoniefferia) plena

Efferia plena (or Pogoniefferia plena), is a very similar species, but note only three of the segments have much white, and only the first has the long hair swept to either side. Also notice the femora and tibiae are closely covered with very short yellow hairs, which serve to dampen the contrast between the black femora and red tibiae. Additionally the bulb has a lot of those same short yellow hairs, so it does not look as black and shiny as on E. nemoralis.


This is the female E. plena, perhaps impossible to tell for certain from the female E. nemoralis. But I fancy that the yellow hairs on the legs are again obscuring somewhat the red-black contrast. This female is laying her eggs in dried sedge flowers. Both E. plena and E. nemoralis are found in damp overflow areas and perhaps it makes sense to lay the eggs above the ground of a field that might flood in winter, so the larvae can wait for dryer conditions before dropping down.



[back to the top]


Efferia (Pogoniefferia) prairiensis

Here is an easy one: only one white segment, a yellow-orange mystax, and bright orange tibiae. This is Efferia prairiensis (or Pogoniefferia prairiensis), a robber of remnant prairies, but also, on Crowley's Ridge, found in grassy fields.


The female E. prairiensis has a short ovipositor, rather short wings, the bright orange legs, and, in good light, an overall golden color.



[back to the top]


Efferia (Pogoniefferia) pogonias

Efferia pogonias (or Pogoniefferia pogonias) comes out in the fall, in September, and like many late season robbers, is black in color, perhaps to absorb more sunshine late in the year. They can be quite common along dirt tracks through grassland or open woodland. The yellow mystax, black color, two white segments on the abdomen, and late-season appearance will identify the male. This one is eating a large skipper, one of the cloudywings.


The female E. pogonias is also dark with a yellow mystax. Both sexes are excessively hairy, perhaps another defense against late season weather.


The female uses its short ovipositor to shove its eggs under lichens and leaf litter, or in this case under stones and pebbles in the loose soil.



[back to the top]


Triorla interrupta

The last of the Efferids on Crowley's Ridge is generally known by its new genus name of Triorla interrupta. It is a medium to large-sized robber that lands, sometimes, in low bushes, but usually lands on the bare ground of a path and flies up ahead of you as you walk, catching the grasshoppers you have chased up. The side of the abdomen has a distinctive pattern: The first two segments are dark brown on the anterior half. The next two segments are entirely dark brown. Then the next three are pale, with sort of a yellowy cast. The genitalia on this male do not turn up, as in other Efferids, nor are the legs contrastingly colored.


The female Triorla has more or less the same marking on the side of the abdomen, and ends the abdomen with a short ovipositor. Also, as you can see in this picture, there is a broad dark mid-dorsal stripe on the thorax of both sexes, which is usually quite noticeable.


Triorlas are fearless big-game hunters, often catching dragonflies much larger than they are, or big grasshoppers which have dangerous spiky legs. Their technique is to grab big prey in the air with their strong legs and bring it to earth, and then, often lying on their side (since they need all six legs to hold their prey—note how this one is holding the grasshopper's hind legs from moving), maneuver it around so they can stab it in the back of the head.


In this picture of Triorla with an enormous horse fly, I see the robber fly with its white beard, and can't help thinking of the famous picture of Hemingway standing on the pier with the giant marlin he has just caught hanging up behind him.



[back to the top]


Mallophora orcina

There is another bumble bee mimic that is not related to Laphria, and that is, Mallophora orcina. These differ by being, generally, out in fields instead of in or on the edge of woodland. Also (a feature that can be seen with close-focusing binoculars), they have a slender style at the tip of the thicker part of the antennae (Laphria antennae lack that style). Also note the pattern: the front part and the back part of the thorax have yellow hair, while the central part is bare. (Sometimes the yellow wears off on old Laphria, but that leaves a round bare patch, like male pattern baldness.)


You can see in this shot what a fine bumble bee mimic Mallophora orcina is.



[back to the top]


Proctacanthus duryi

There are four members of the genus Proctacanthus (large powerful creatures with long slender abdomens sometimes called Giant Robber Flies) on Crowley's Ridge. This one, recognized by the pale hairs on the very back of the thorax, is Proctacanthus duryi. They are found on sandy beaches (hence their pale sandy color). I watched this one, flying low along the water's edge, suddenly stoop down and pluck up this Bronzed Tiger Beetle.



[back to the top]


Proctacanthus hinei

This distinctive reddish species is Proctacanthus hinei. It occurs in summer and fall on open sandy beaches along the St. Francis River (such as, by the town of St. Francis, in Clay Co.). It also occasionally occurs in abandoned quarries. The species is notorious for being skittish and unapproachable. It appears very much like a red Polistes wasp flying low and fast over the beach. If you're lucky, it will land somewhere in view.


The females favor loose powdery sand for ovipositing, and instead of digging, they just plunge their abdomen into it. Males set up territories in these powdery areas, to waylay the females when they arrive.



[back to the top]


Proctacanthus milbertii

The commonest Giant Robber on the Ridge, Proctacanthus milbertii, is a big rather plain robber of open fields with bushy vegetation, where it lands on the dirt path or on top of the bushes, and is particularly fond of grasshoppers, but will eat anything it can catch. It differs from Proctacanthus duryi in habitat, in being somewhat larger, and in having black hairs at the back of the thorax. It's a late summer and fall species.


Here is the female P. milbertii. To compare it with the next, similar, species, note particularly the brown marking on the thorax, the gray femora, the pale tan abdomen.



[back to the top]


Proctacanthus nigriventris

The fourth Proctacanthus on the Ridge is the very dark Proctacanthus nigriventris. It appears side by side with Proctacanthus milbertii in rank bushy fields over sandy ground. I have found it on Hatchie Coon Island along the St. Francis River in Craighead County. It differs from P. milbertii by appearing much darker. Note especially the black marking on the thorax, the sooty black femora, and the blackish abdomen. It tends to stay down lower in the vegetation.



[back to the top]


Promachus bastardii

There are three robbers in the genus Promachus. The first is Promachus bastardii (no doubt there is a story in the naming of this species). It is the species I used in the section "What is a Robber Fly?" at the beginning to show the features of a "typical" robber fly. It is at the large end of being medium-sized, and is by far the commonest and most widespread robber, found in every possible habitat, and catching any prey it finds (though it is especially fond of honey bees). Identification of the male is easy: the yellow facial hair, the silvery-white hairs at the very tip of the abdomen, and (this is important to separate it from its close relative) the solid black on top of the abdomen, easy to see in this photograph.


The female P. bastardii is a little trickier to separate from some other flies, but it is a generally dingy fly with a short lumpy ovipositor, and, in common with the male, has yellow facial hair and black on top of the abdomen. If you are walking through a field that has several P. bastardii in it, from time to time you will hear a loud, extended buzz, and it will be a male hovering stationary, sometimes for several seconds, in front of a female, and this is a prelude to mating.



[back to the top]


Promachus fitchii

Promachus fitchii is the very attractive close cousin of P. bastardii, and features the same yellow facial hair and white-tipped abdomen on the male. In spite of the brighter coloring, and the eyes that can't make up their mind whether to go green or red, they can sometimes be a little tricky to separate. But notice: on P. fitchii, the top of the abdomen is not solidly black. This species only seems to occur in open meadows.


The female P. fitchii is, similarly, a blonde version of the female P. bastardii, but, once more, without solid black on top of the abdomen. The non-contrasting femora-tibiae will help separate it from Efferids.



[back to the top]


Promachus hinei

The third Crowley's Ridge representative of the genus is Promachus hinei (not to be confused with Proctacanthus hinei above). It is a spectacular robber, as big as the Giant Robbers, with bright red legs, and a handsomely banded abdomen. This fall-occurring species is much more of a woodland species than the other two Promachus, but it is also common in rankly overgrown fields.


In certain lights the eyes become noticeably purple. This one has caught a bumble bee.


The female Promachus hinei has a longish thick ovipositor.



[back to the top]