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Dance-flies and the Long-legged Flies

(Families Empidae and Dolichopodidae.)

The flies of the family Empidae are rather slender, sordid, uninteresting-looking creatures, usually of small size. They are sometimes called dance-flies, because they are often seen in swarms in the woods flying up and down with a dance-like movement. By "dance-like" is not meant the slow, gliding movement of modern waltz, but the robust up and down backwoods jig movement. It is a very large family comprising more than eleven hundred species. They resemble the robber-flies somewhat in form and also in habits since they are predatory and capture other insects. Some of them have the curious and as yet unexplained habit of carrying little silken webs with them when they fly, although no one really knows how they spin these webs or where they get them. Their use has also been a mooted point, but it has been suggested that they act as parachutes or aid in the capture of their prey.

Aldrich has studied what is probably this same phenomenon. He finds that the males of an Empis carry little oval masses larger than themselves, which are really not composed of silk, but of bubbles of a viscid substance. The purpose of this structure is to attract the female. Aldrich says, "When numerous males were flying up and down the road it happened several times that a female was seen to approach them from some chokecherry blossoms nearby. The males immediately gathered in their path and she with little hesitation selected for a mate the one with the largest balloon, taking a position upon his back * * * The pair would settle down toward the ground, select a quiet spot * * * here she would continue to hold the male beneath her for a little time. The male meanwhile would be rolling the balloon about in a variety of positions, juggling with it, one might almost say. After the male and female parted company the male immediately dropped the balloon upon the ground and it was greedily seized by ants." It seems probable that this observation by Professor Aldrich will explain all of the cases in which dance-flies are seen to carry such structures.

Many of the species do not capture their prey by flight, but run rapidly about on the ground catching other insects with their front legs. The female, as in certain other predatory groups which we have mentioned or will mention, resents the approaches of the male, and if he be incautious seizes him and dispatches him at once. The only time at which he can with safety make his advances is when she is busy eating some other insect.

The larvae of the dance-flies are cylindrical, and live in the earth under leaves and in decaying vegetation. They are said to be probably carnivorous. One species was reared, however, from human faeces in the course of certain investigations made by the writer on the subject of the flies which may carry the germs of tpyhoid fever, but of course they may have been preying upon other insects which were feeding in this substance. The pupae are said to be free and to possess two points at the front end.

The Dolichopodid flies are rather small and rather slender species, usually greenish or bluish in color and more or less metallic. Their wings are clear, or they may be dusky or obscurely banded. They possess long legs, from which fact they are called by Comstock "the long-legged flies," which, however, does not seem to be a good popular term, since there are other flies with much longer legs. Just as with the dance-flies, these creatures are predatory and capture small flies of other groups, and even soft-bodied worms. They are found in damp places upon the leaves of aquatic plants, and some of them are able to run rapidly over the water. The proboscis is short and not as strong as with the robber-flies. It is a large group, and about twelve hundred species are known. Many forms occur in this country. With none of the species, however, has a good full life history been studied out. The larvae live in the earth or in decaying vegetable matter; some are found under the bark of trees or in flowing sap. They form a cocoon and the pupa has two long breathing tubes on the back of the thorax.