Strange Flies on a Fish Carcass

Spring has been dragging her heels here in northeast Kansas, USA, but my partner and I managed a brief outing to take advantage of a certifiably warm and sunny day this past Saturday. Finally, there were a few insects to be seen, even if they were mostly non-biting midges, a few other flies, one butterfly, and several Eastern Boxelder Bugs. Heidi spotted the remains of a large fish just off the trail, and I noticed some small, unusual flies dashing over the head of the dead piscine. I managed a few images, and saw that I had something unfamiliar and pretty interesting.

One female (top, left) and two male waltzing flies

The location of our short hike was the Mill Creek Streamway Park in Johnson County, Kansas, north of Shawnee Mission Park. We were on Nelson Island in the Kansas River for most of the time, and that is where we found the fish remains. The overstory was mostly cottonwood trees, the understory just beginning to green up.

A blow fly and at least two waltzing flies

There were the obligatory blow flies on the fish, too. I suspect the deep metallic blue insects were either Phormia or Protophormia. Without collecting specimens, fly identification is problematic. It comes down to patterns of setae, the color of thoracid spiracles, and other miniscule characters that are not usually visible in mere photographs of wild, mostly moving targets.

The smaller flies were another matter. They were so unique that once I returned home I was immediately able to identify them to species. More importantly, I learned about their bizarre behavior. About twice the size of your average kitchen-inhabiting "fruit fly," Drosophila sp., these flies were slender and long-legged, the better for running around and chasing each other, as was apparent in the field.

Male waltzing fly

Surprisingly, my hunch that they were members of the "skipper flies" family Piophilidae was correct. The most famous members of the family are the cheese skippers, named for the "jumping" maggots that infest rancid cheese, "skipping" away from danger by grasping their rear end in their mouths and suddenly releasing it, catapulting themselves vertically and horizontally. As a whole, the larvae of most species feed on decomposing fatty tissues in carrion, often after the corpse begins drying out.

Female (top) and male waltzing flies

The flies I had been observing were "waltzing flies" of the species Prochyliza xanthostoma. Their most distinguishing aspect is the sexual dimorphism between males and females. Males have greatly elongated heads, with a torpedo-shaped face and remarkably long atennae originating from the tip of the cone. Females are more....demure, or at least less comical in appareance, with a head of more normal dimensions.

Why the long face on the males? Stephen A. Marshall, in his epic tome Flies: The Natural History and Diversity of Diptera, explains:

"One of my favorite piophilines is Prochyliza xanthostoma, a Nearctic species that appears in abundance in early spring when melting snow exposes the carcasses of winter-killed animals. Males of P. xanthostoma have long antennae and a greatly elongated head that is used in ritual male-male battles for territory, during which these striking insects stand on their hind legs and engage in vigorous head-butting contests."

Dr. Erica McAlister, in her fascinating book The Secret Life of Flies, also notes the ferocity of these sparring matches:

"The males in another of the group, the waltzing fly, Prochyliza xanthostoma, have quite wonderful heads - almost conical in shape with very large and thick antennae - and they feed on animal carcasses. As their name suggests the males dance to woo the females. But they also have to defend their territories (carcasses) from other males and can have the most amazing fist fights - they really reign down punches on each other.

A female ignoring two competing males

I am now regretting not to have taken a video or two while I was there, though Heidi had long since made her way far ahead of me on the trail. Should I encounter waltzing flies again, I won't make that error of omission. Meanwhile, you might want to search for them. The species occurs over most of North America, especially the eastern half of the continent. If you are sufficiently intrigued, this Wikipedia article goes into extraordinary detail about the behavior and biology of the species.