In summer the empire of insects spreads.
Last week I noticed a medium-sized robber fly noisily flitting about the garden. It landed on a cedar twig on the garden fence and I leaned in for a closer look.
It was distinctive in the bright white patch near the tail tip, something I noticed from a few feet away. As I leaned in I could see the white was actually a pair of tufts of “hairs”. I figured this might be an easy one to identify so off I went for a look in Bug Guide. Sure enough, the white tufts are diagnostic of a couple of species. After reading more about the ranges and other characteristics, I decided this was a male Promachus bastardii.
Males of this species are characterized by white tufts of hairs along the edges of the abdominal segments, black on the dorsal surface of the abdomen, and the distinctive white tufts at the tip of the abdomen.
Robber flies have several features that set them apart from other flies…large size; eyes set wide apart for better depth perception; the hairy face or the mustache of bristles called the mystax (theorized to protect the eyes during struggles with large prey); the arching thorax containing superior flight muscles (an advantage for rapid lift and flight to snag fast-flying prey out of the air); a sharp beak, partially hidden by the mystax, which is used to stab their victims, injecting toxins and digestive fluids, which allow the predator to kill the prey and suck the juices out; and long, spiky legs with what look like talons at the tips for catching and holding their prey. This is one of the so-called Giant Robber Flies, due to the group’s large size. This species is actually one of the medium-large ones in the group, and this male (which is smaller than a female) was a little over an inch in total length. I found only one reference to a common name for this species, Bee Killer, for their habit of preying on various species of Hymenoptera. The scientific name immediately catches your attention, and translates to “the bastard’s champion”. Since the species was named in 1838, it is probably lost as to the reason, but I am curious. Perhaps it was named by a fan of bees.
A few days later while strolling in the yard, camera in hand, I heard a loud buzzing noise behind me and turned to find the same species of Robber Fly (perhaps the same individual male) subduing a prey item, a small species of bumblebee. These bees are common at my wildflowers, especially the Rosinweed (Silphium sp.) that is blooming now along the driveway. The fly had already stuck its beak into the bee, which soon quit struggling.
Watching this drama unfold, I admired the strength and apparent agility that is needed to catch and subdue this type of prey. And I was thankful this guy is not two feet long…few creatures in these woods would be safe from such a aerial predator.
ANOTHER POLLINATOR PREDATOR…
After writing this post, I found another peril for pollinators in this jungle of a yard. An orb weaver spider had strung a giant web across a main pathway through the yard and snagged one of the Eastern Tiger Swallowtail butterflies that have been feeding on the Joe-Pye Weed that is now blooming. A jungle indeed.
Did not know that there was such a fly. Sad that he can kill bees. Have they been around??
This is a fairy common group of predatory insects. They are quite varied, including some species that are excellent bee mimics.
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