The Mating Activities of Balloon Flies

Abstract

IN 1875, while visiting in the Swiss Alps, Baron Osten-Sacken made the first recorded observations on what we now call balloon flies. On sunny days, between the hours of about 9 and 10 o'clock, he noticed brilliant, silvery flashes among the sunbeams which penetrated the shadows of the fir forest. When he concentrated his attention on the flashes he concluded that they must be produced by insects, so he determined to catch some and added an extension to his net handle. The white flashing objects seemed to dodge with agility and it was only after several swings that he was certain he had netted one of them. However, when he looked into the net, he was at first astonished to see only an inconspicuous dull-colored fly, much smaller than he had anticipated. He assumed that this one had been captured by accident, along with the one he had tried for. So he examined his net carefully and it was only then that he noticed the white, filmlike packet of sparkling material on the gauze. He tried to pick it up, but it was so light that his breath carried it away. He pursued it and finally eased it into a vial. This was the first of the so-called fly-balloons to be taken. To make certain that the balloons actually were associated with the flies, OstenSacken caught one specimen after another, always with the same results. Each time there was the balloon in the net along with the fly. And examining the flies with his hand lens, he observed that invariably they were males. (See Fig. 1.) It was Becker (1888) who described this fly that Osten-Sacken had discovered, and he named it Hilara sartor. Hilara is a genus of the family Empididae which is made up of the dance flies, so called because of the habit indulged in by many species of dancing on the wing in assemblages which sometimes reach huge proportions. All of the balloon-making flies belong to the Empididae. Becker was interested in natural history as well as taxonomy, so he joined Osten-Sacken in speculation as to what the significance of the balloon might be. In fact he went to observe the flies in their home environment, but they flew too high for him to get close enough to make accurate observations. As a result he reached the erroneous conclusion that the males carry the white structures on their backs. He also supposed that these structures serve as decorations which attract the attention of the females, and of course in this he was not far wrong. That same year Mik (1888) also went to see these flies in action, and apparently he had either better eyesight or a taller stepladder than Becker because he was correct in insisting that the flies carry the balloons below their bodies, where they are suspended by the legs. He observed, moreover, that these structures are considerably flattened, this flattening being much more pronounced than in any of the later-discovered species. This observation led him to propose the fantastic theory that the male hilaras use the flattened objects as aeronautical surfboards on which to cavort among the sunbeams. The next entomologist to speculate on the significance of the balloons was Verhoff (1894). He concluded that they must serve as warning signals to birds and predaceous insects. There is no evidence that these flies are distasteful to birds; on the contrary I have seen birds attack swarms of related species of empidids. In regard to the second part of Verhoff's