A Diversion to Utah, Part 2: Robber Flies and Velvet Ants

On our trip to southern Utah in July of this year, I had hoped to add to my photo collection of Asilidae (robber flies), as they are such impressive and photogenic predators. I have never had much luck photographing live velvet ants (family Mutillidae), but there were SO many of these in our Burr Trail camp that it was downright easy to get excellent close-ups of them.

The recent monsoonal rain had brought out quite a lot of local flowers, and perhaps that's why the robbers were out in good numbers in most places we visited.

Robber flies are often easy to photograph as they have some loyalty to a perch from which they search for prey items flying past. The fly will typically ambush its prey in mid-air, and administer a bite which delivers a neurotoxin to paralyze the prey, combined with proteolytic enzymes to digest the prey's insides, making a soupy meal to be sucked up at its leisure with the proboscis of the fly.

Efferia sp., male. He was lurking in plain sight on some dried-up mud at the little cattle pond (that we visited two years ago) east of our campsite on the Burr Trail. "Efferus" comes from the Latin meaning "wild" or "savage" - fairly appropriate, I guess, although that description could apply to just about any predator, from a robber fly to a mountain lion!

Efferia sp., female (note the long thin ovipositor at the terminus of the abdomen). As I crept up to her (on the opposite side of the cattle pond from the male), she crouched down, stretching her forelegs out, but did not fly off. It seemed like a very deliberate behavior. I wonder if there is any meaning to it, past just making her appear slightly smaller.

Efferia sp. female, with fellow asilid as prey. No family loyalties here.

The "pond Efferia male". Side view. The fuzzy mass of hairs protruding from the face is the "mystax" (meaning moustache) which is thought to protect the fly's face from wildly struggling prey.

Large asilid with prey, in the creek bed which passes by our Burr Trail camp site.

A smaller asilid species, to be determined, perched on the concrete bridge over the creek.

An almost-all-black robber fly, Ospriocerus sp. (note the red on the abdomen). This is a large species, like the Efferia seen on this trip - around 1 inch long. 

Another Ospriocerus sp., east on the Burr Trail, in an open windy area.

The sandy substrate combined with plenty of flowering plants following the rains seemed to create the ideal conditions for velvet ants around our Burr Trail camp site. Four species were seen within and close by our camp. I have not been able to conclusively identify any of them yet.

This species, with a black abdominal tip, came in orange and yellow forms. I'm tentatively guessing Dasymutilla scitula?

Digging in the soft sand around the camp site.

A very small but beautifully clothed female - actively searching for larvae of ground-nesting host species.

This small species, as well as the other two diurnal species we saw, was searching for the nest chambers of ground nesting bees or wasps. The female will dig her way into a nest, lay an egg by the host larva, and then the egg will hatch and consume the host.

This was the larger species, which might be Dasymutilla californica, but still uncertain. This female is next to a hole she dug out.

Busily digging, digging, digging. At least half a dozen females of this species were active in the late afternoon around our camp site. A couple males were present also (presumably of the same species, although with several species present, and the fact that male and female velvet ants of the same species often do NOT look alike at all, it's not certain which species the males belonged to).

Some of these females were very worn looking, or even had what appeared to be dried mud on them.

One of many neat holes dug in a small area with many velvet ants actively digging around. This hole was around 1/4 inch in diameter.

A male. Males are winged, but do not sting. Females of many velvet ants are wingless. Females can deliver a powerful, painful sting!

A large winged mutillid, probably in the genus Sphaeropthalma, which came to the black light after dark. It was around 20 mm long.