Dancing Flies

by Carl Strang

A program had me at Kline Creek Farm early one morning last week. While waiting at the parking lot for others to arrive, I noticed a swarm of small flies engaged in what I took to be a courtship dance. They were in the light at the edge of a tree-cast shadow, and they remained in the vertical plane defined by that edge, within an altitude range of about 5-8 feet, the zone parallel to the ground and around 20 feet long. Each fly followed a roller-coaster or sine-wave flight path, turning around when it reached each end of the zone.  They were going too fast to follow easily, and I had no net with me. I took a few photos.

The flies could be seen clearly only when backlit. Each bright dot is a different fly.

The flies could be seen clearly only when backlit. Each bright dot is a different fly.

The flies in focus in each photo had similar shapes. Here are two examples:

There is a large spot in the middle, and two lines at each side. The shutter speed is 1/1000 second.

There is a large spot in the middle, and two lines at each side. The shutter speed is 1/1000 second.

Often there were additional projections above and below.

Often there were additional projections above and below.

There is a family of flies called dance flies, the Empididae. They are famous in ethological circles for the nuptial gifts offered by males to females. In some species the pattern is regarded as more primitive, and the gift is a prey item. In others the male wraps the prey in silk. At the other end of the spectrum are species in which the male creates a balloon of silk, and this has replaced the prey entirely. Were the photographed flies members of this family? I do not know. Empidids have bulbous thoraxes, which would account for the bright central spot in the photos. The abdomen is thin, and the long central line could be a highlight, with the wings similarly indicated by the lines to each side. The thicker line below could be the dangling long legs, or perhaps the long proboscis that some species of empidids possess, if these were in fact empidids. Whatever they were, they were fascinating to watch. The shadow-edge convention provides a standardized meeting site, reminiscent of hilltopping butterflies. I found the up-and-down motion of the individual flies hypnotic, the experience delightful.

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