A White-tailed Robber Fly from League City, Texas

BugsInTheNews is a VIEWER-PARTICIPANT WEBSITE. This article by Jerry Cates and Mark Turvey,  first published on 31 July 2011, was revised last on 7 July 2012. © Bugsinthenews Vol. 12:07(09)


Promachus bastardii Macq.: frontolateral; Mark Turvey, League City, TX --- 31 July 2011


On 31 July 2011, Mark wrote:

“Hi, Jerry

Shot these photos of a strange-looking fly back on May 11th. He would move occasionally but let me get close for a few shots. Had never seen one of these….

Any idea what it is?


The robber flies comprise a highly interesting family (Asilidae; presently divided into seven subfamilies) of arthropods in the insect order Diptera (true flies). More than 7,100 species have been described.

Mark’s specimen is rather well-known as the white-tailed robber fly, though only the male of the species has the fluorescent white marking on the distal portion of its dorsal abdomen (the female’s abdomen is practically devoid of markings, being a darkened coloration throughout). Other species of robber flies display this same or almost-the-same marking, but — one gathers from a quick perusal of the literature — none except Promachus bastardii (Macq.) has it in combination with a robust, reddish-orange facial mustache, orange tibia and metatarsus on leg I, orange tibia on leg II, and the pale white markings on the lateral abdomen.

This species was first described in 1838 by the French entomologist Justin Pierre Marie Macquart (1776-1855). This gentleman was the younger brother of a celebrated ornithologist, and of another brother (age relationship unknown) whose botanical collection was at the time without peer. Being a sibling of two outstanding naturalists set the bar of scientific achievement high, and he more than rose to the challenge.

Macquart chose the epithet Promachus for the generic name of this insect, using the Greek προμαχιζω, “fight in front as a champion,” out of respect for the vicious, and generally victorious, behavior of these flies when engaging in predatory attacks. Macquart’s basis for selecting bastardii as the insect’s specific name is obscure.

One of the most frequently used common names for this fly is the white-tailed robber fly, but the name is misleading for obvious reasons. Though unwieldy, the “orange-faced, white-tailed robber fly” would be more descriptive.

Promachus bastardii Macq.: left laterodorsal; Mark Turvey, League City, TX --- 31 July 2011

left laterodorsal

Promachus bastardii Macq.: left laterodorsal; Mark Turvey, League City, TX --- 31 July 2011

Left lateral

Promachus bastardii Macq.: dorsoposterior; Mark Turvey, League City, TX --- 31 July 2011


Robber flies are characterized by their spiny legs that sport stout femurs and tarsi with two claws; a mustache of dense bristles facially known as the mystax, positioned above the mouth and below the vibrissae (the stout bristles on the face below the antennae); three simple eyes (ocelli) in a relatively deep depression between two large compound eyes; and short, 3-segmented antennae that in some species sport a bristle-like structure (the arista).

Robber flies are equipped with a short, but strong proboscis which is used like a sword to pierce the integuments of their prey. The proboscis is further equipped with an orifice, distally, through which salivary secretions are injected into the prey. These secretions contain neurotoxic enzymes that paralyze and incapacitate, and proteolytic enzymes that digest animal tissue. Once the robber fly attacks and subdues its prey (which often includes insects of its own family, and even its own species; robber flies are notorious as true cannibals), it remains attached via its proboscis until the prey’s internal tissues have been digested to a liquefied state, whereupon it sucks the “nutritious” (!) liquid up through the same orifice that delivered its salivary secretions, to consummate its grand, though — from a human perspective — disgusting, feast.

Promachus bastardii Macq.: frontal; Mark Turvey, League City, TX --- 31 July 2011


White-tailed robber fly (Promachus bastardii Macq.): dorsum; Mark Turvey, League City, TX --- 31 July 2011


These flies are known by bee keepers as “bee-killers,” as they seem particularly fond of honey bees. Perhaps the latter’s diet of honey and pollen makes their internal tissues unusually sweet. In any case, the presence of numerous robber flies near an apiary spells bad news for bee keeping, as most experienced bee keepers will attest.

Though the beak of the robber fly is hard enough to easily pierce the skin of humans and other mammals, it is not known to behave so unseemly toward its mammalian neighbors. That’s a good thing, as while we humans walk through a wood or meadow where these flies are present, it is not unusual for one of them to land on a hand or arm and perch there — harmlessly, thank goodness — to rest a moment before flying away to search for something to eat in the insect world.

And, lest we forget, though these flies are ruthless in their behavior toward insects, and sometimes they end up eating insects (like bees) that benefit mankind, they also do much more in the way of good for us by reducing the populations of truly harmful insects in our midst.

References to Scientific Literature:


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