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Stone-flies — Part 2

Although dragon-flies are frequently very abundant in swampy regions and about ponds, there are times when they swarm in enormous numbers. Koppen, a German entomologist, has published a chronological account of the records of dragon-fly migrations, from 1494 to 1868. Such migrating swarms seem to have been more frequently noticed in Europe than in this country, but several have been noticed in the United States. For example, Mr. A. H. Mundt, of Fairbury, Illinois, says that between the hours of 5 and 7 P. M., August 13, 1881, "the air for miles around seemed literally alive with these dragon-flies (AEschna heros) from a foot above ground to as far as the eye could reach, all flying in the same direction, a southwesterly course, and the few that would occasionally cross the track of the majority could all the more easily be noticed from the very regular and swift course they generally pursued; but even these few stray ones would soon fall in with the rest again. Very few were seen alighting, and all carefully avoided any movable obstacles." This migration was probably caused by the very dry season which had resulted in the drying up of ponds and swamps, and it is probable that other similar recorded migrations have arisen from the same cause.

Among the insects killed by dragon-flies there must be, of course, some mosquitoes, although the beneficial work of these insects in this direction is greater in the larval stage than in the adult. Dragon-flies are day flyers, but in cloudy weather and toward evening many mosquitoes are undoubtedly killed by them. Dr. E. A. Mearns, U. S. A., (quoted by Beutenmuller) states that at Fort Snelling, Minn., mosquitoes appeared in vast swarms, and were soon followed by large numbers of dragon-flies after which the mosquitoes were considerably reduced in numbers. Dr. Robert H. Lamborn, noticing in the Lake Superior region the activity of dragon-flies in this regard, years later offered a prize for the best essay on the artificial multiplication of dragon-flies for the destruction of mosquitoes and house-flies. The prize essays by Mrs. Carrie B. Aaron, Mr. Archibald C. Weeks, and Mr. William Beutenmuller were published in 1890 in a very readable and valuable book, but as might have been foreseen the practical value of Dr. Lamborn's suggestion was not substantiated.

The eggs are laid either in the water or are inserted in the stem of some aquatic plant.

In the dragon-flies of two families there is no apparatus for the insertion of eggs into plant stems, and they are therefore either dropped loosely in the water or attached to submerged objects by means of a mucilaginous substance which surrounds them. With others, however, there is a curious modification of the end of the body. The sides of the vulva are pointed or roughened, and cut into plant tissue so that the eggs may be pushed into the cuts. Here the female gradually crawls down the stem of a water plant until she is often completely submerged. She is always incased with an air film so that she can continue to breathe under water to a certain extent. The number of eggs is variable, but is usually large. Some very curious Hymenopterous parasites live in the eggs of dragon-flies.

When the eggs hatch the young immediately begin an active, predatory life under the water, feeding upon other aquatic insects. This food habit is continued throughout their larval or nymphal existence, and as they grow larger they are able to over-power larger and larger insects and even small fish and other aquatic animals. They will kill others of their own kind, and nymphal dragon-flies have been seen to catch and destroy adults in which the wings were not yet expanded. They molt probably a number of times, but the exact number has not been recorded for any species, so far as I know. The most peculiar feature of the larva or nymph is the strange modification of the mouth. There has been a backward growth of the lower lip and this has become hinged so as to form a long, hinged apparatus with sharp teeth at its extremity. It can be folded to cover the lower face like a mask. The structure has been called a mask, and when it is folded the head of a dragon-fly larva seen from the front looks like that of a bulldog. It is more innocent looking than that of a bulldog simply because we know what a bulldog can do, but the moment that the larva approaches near enough to its prey the innocent looking mask is unfolded and darted out, and the probably unsuspecting aquatic insect or small fish is seized by the teeth at the extremity and drawn back into the mouth.