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Stone-flies — Part 3

Dragon-fly larvae breathe in a peculiar way modified to some extent with the members of the different families. The rectum is furnished with very many tracheal branches, forming numerous loops and even penetrating the walls of the intestine. Water is sucked into the rectum, and these "rectal gills," as they are termed, derive their oxygen from this water. This same feature affords with some species a means of locomotion, for this water which is sucked in for breathing purposes may be ejected violently, the effect of which is to send the larva ahead. This same principle has been used in certain mechanical toys, and applications have even been made for patents on a similar method of propulsion for vessels. With some there are external abdominal gills, both lateral and caudal, while when the nymph becomes full grown and is ready to leave the water breathing is taken up by certain obscure spiracles. There is still some doubt as to the exact method by which they begin to breathe air after leaving the water, and it is a subject which will bear much further investigation.

When the nymph becomes full grown, it has changed its form from a rather slender creature to a broad and flattened one, not resembling the slender bodied adults in the least. It crawls out of the water on the bank upon the stems of water plants or upon the rocks, and later its skin splits down the back, and the adult dragon-fly emerges. Empty skins of these nymphs are very common objects about watercourses.

Rather more than two thousand species of dragon-flies have been described, and of these something less than three hundred inhabit the United States, of which about two hundred and twenty-five species are peculiar to this country. According to Kellicott, about one hundred species are found in the State of Ohio, and Williamson thinks that even more are to be found in Indiana. In many places dragon-flies are disappearing, owing to the drainage of their breeding places.

On account of the beauty of the adults and the interest attaching to their habits, they are becoming favorite subjects for collections, and there are now a number of earnest students of the Odonata in this country. The recent death of the great master of dragon-fly science, Baron de Selys-Longchamps, of Belgium, December 11, 1900, has called renewed attention to this fascinating group.

The nymph dragon-flies are well adapted to aquarium study. They are easily collected and easily kept. The debris at the bottom of ponds can be brought up with a rake, and the nymphs thus collected placed in a bucket and carried home to the aquarium, which should be furnished with sand and aquatic plants. The best time for collecting them is in the spring and early summer.