Relations of a Late Autumnal Dragonfly (Odonata) to Temperature

Abstract

In the Botanical Garden of the University of Pennsylvania, in Philadelphia, is an artificial pond of oval shape, i io feet long,. 55 feet wide (33 x 17 metres) with an area of about 4,070 square feet (378 square metres). It was made some thirty years ago on a former gravel and clay bank, at least io feet higher than the present level of the ground. The pond bed was excavated in such a way as to leave borders sloping down toward the water on three sides; on the north, the greenhouses of the Department of Botany are within 50 feet. The sloping borders are rock garden, while higher up are trees now in their twentieth to thirtieth year. A weeping willow hung over the water for many years but has passed away. In the pond itself many forms of water lilies (Nvrnpliaea), lotus, Elodea and other plants have rendered it a very attractive spot. Two or more species of gold fish, minnows, occasionally Pectinatella, and a wealth of other aquatic animals have made it a prolific source of material to the zoologists of the University. Here in the autumn the wide-spread North American dragonfly, Syrnpetruin vicinufn Hagen, is regularly to be found. The bright red abdomen of the male lends a brilliant touch of color on sunny days of September and of the two following months. The duller brown of the female ordinarily renders her less conspicuous. This species has the habit of egg-laying by pairs, that is, the male and female fly together in this operation, the claspers at the hind end of his abdomen holding her head, and so united they follow the pond's margin, or cross its expanse, her abdomen every now and then dipping into the water in which her eggs fall loosely. Since this pond lies within three minutes' walk from either of the two buildings which have housed the zoological department during the pond's existence, it has been possible to avail one's self of the often limited period between classes at the middle of the day to visit the pond and to note the presence or absence of this Sympetrumn. In looking over my diaries, I find that I now have such records for twenty four years: I898-i900, 1902, 1904-07, 1910-1925. I believe that there are few observations of this nature on the same species, at the same limited locality, for an equal number of years, in existence. It has therefore seemed worth while to tabulate these observations and to compare them with the meteorological conditions for the