This spider carries within its diminutive abdomen a secret which will possibly serve to vex subtle intellects for a long time to come; for it is hard to believe that merely by mechanical force, even aided by currents of air, a creature half as big as a barley
Facts and Thoughts about Spiders. 185
grain can instantaneously snoot out filaments twenty or thirty inches long, and by means of which it floats itself in the air.
Naturalists are now giving a great deal of attention to the migrations of birds in different parts of the world: might not insect and spider migrations be included with advantage to science in their observations? The common notion is that the gossamer makes use of its unique method of locomotion, only to shift its quarters, impelled by want of food or unfavourable conditions--perhaps only by a roving disposition. I believe that besides these incessant flittings about from place to place throughout the summer the gossamer-spiders have great periodical migrations which are, as a rule, in-visible, since a single floating web cannot be remarked, and each individual rises and floats away by itself from its own locality when influenced by the instinct. When great numbers of spiders rise up simultaneously over a large area, then, sometimes, the movement forces itself on our attention; for at such times the whole sky may be filled with visible masses of floating web. All the great movements of gossamers I have observed have occurred in the autumn, or, at any rate, several weeks after the summer solstice; and, like the migrations of birds at the same season of the year, have been in a northerly direction. I do not assert or believe that the migratory instinct in the gossamer is universal. In a moist island, like England, for instance, where the condition of the atmosphere is seldom favourable, and where the little voyagers would often be blown by adverse winds to perish
186 The Naturalist in La Plata.
far out at sea, it is difficult to believe that such migrations take place. But where they inhabit a vast area of land, as in South America, extending without interruption from the equator to the cold Magellanic regions, and where there is a long autumn of dry, hot weather, then such an instinct as migration might have been developed. For this is not a faculty merely of a few birds: the impulse to migrate at certain seasons affects birds, insects, and even mammals. In a few birds only is it highly developed, but the elementary feeling, out of which the wonderful habit of the swallow has grown, exists widely throughout animated nature. On the continent of Europe it also seems probable that a great autumnal movement of these spiders takes place; although, I must confess, I have no grounds for this statement, except that the floating gossamer is called in Germany "Der fliegender Summer"--the flying or departing summer.
I have stated that all migrations of gossamers I have witnessed have been in the autumn; excepting in one instance, these flights occurred when the weather was still hot and dry. The exceptionally late migration was on March 22--a full month after the departure of martins, humming-birds, flycatchers, and most other true bird-migrants. It struck me as being so remarkable, and seems to lend so much force to the idea I have suggested, that I wish to give here an exact copy of the entries made at the time and on the spot in my notebook.
"March 22. This afternoon, while I was out shooting, the gossamer-spiders presented an appearance quite new to me. Walking along a stream