Alluring peacock flies dance on a thistle

By Jeff Mitton
I can hardly resist the lure of flowering thistles.

Initially, their pollinators attracted me, and later I was amused by the larvae of tortoise beetles that spool their feces around their forked tails and brandish their fecal clubs at potential predators.

But most recently, I have been focusing on several species of colorful flies that dance and display on thistles.

The dancing flies are fruit flies, but not the familiar ones in the family Drosophilidae that fly slowly above fruit bowls or form clouds around garbage cans.

These are larger, much more colorful and conspicuous flies in the family Tephritidae, commonly called tephritid flies or, more aptly, peacock flies. About 5,000 species of peacock flies are known; most feed on plant juices, and some are serious agriculture pests.

Paracantha cultaris does not have a common name, although it is common through western North America south to Costa Rica. I find it on spear or bull thistle, Cirsium vulgare, on Flagstaff Mountain, at Sugarloaf and in the Sand Hills of Nebraska. It has brilliant eyes banded in shades of tan and iridescent green highlights. Its thorax is marked with brown circles arranged in lines over a tan background. Its wings have diamond blue spots circled with black and clear chevrons at the margins. It is the most colorful fly I have ever seen.

I first noticed peacock flies while I was looking for pollinators on a bull thistle. The purple top of the flower head had nothing of interest, but the spiked bulb had two species of peacock flies, each larger than Drosophila fruit flies but smaller than houseflies. They ignored me and but continued intensive interactions with one another, meeting head to head, sidestepping in unison, walking slowly about while flicking their wings.

Teams of males occupy a broad lower leaf to establish a lek, an arena where males dance to capture the attention of females. The dance is composed of a series of stylized moves including waving of the front legs, holding their wings at right angles to their bodies and buzzing and flicking their wings. They pump and thrust their mouthparts, producing a droplet of fluid offered as a nuptial gift. Females in attendance signal their readiness by drinking the drop of fluid, holding their wings out to the sides and allowing the male to approach and mate.

After mating, females move to the thistle bulb, where they pierce the bulb with their ovipositor and inject eggs. Adults then drink the sap that oozes from the ovipositor puncture holes. The intense interactions that occur on the thistle bulbs seem to be territorial interactions among females defending oviposition and feeding sites.

The flicking of the wings in the courtship dance has been modified by one species of peacock flies to serve a defensive role against predators. The wings of Zonosemata vittigera have a pattern of stripes suggestive of legs. When a jumping spider approaches, the fly waves its wings much the way it does to seduce female peacock flies. But the spider is fooled by the wing display and pattern, mistaking it for a territorial display of another jumping spider. The hunting spider signals back in apology for unintentionally invading the territory and then backs away.

The wing display effectively terminates the stalking behavior of jumping spiders, but has no effect on other spiders or assassin bugs, praying mantids or small lizards. By altering part of its courtship dance, it mimics the territorial signal of a jumping spider.

I don't think you will see the peacock flies unless you specifically look for them. As the bull thistles flower in June, you will see them dancing on the leaves and maneuvering to defend oviposition and feeding sites on the bulbs.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

August 2011