Facts and Thoughts about Spiders. 187
(the Conchitas, near Buenos Ayres), I noticed a broad white line skirting the low wet ground. This I found was caused by gossamer web lying in such quantities over the earth as almost to hide the grass ad thistles under it. The white zone was about twenty yards wide, and outside it only a few scattered webs were visible on the grass; its exact length I did not ascertain, but followed it for about two miles without finding the end. The spiders were so numerous that they continually baulked one another in their efforts to rise in the air. As soon as one threw out its lines they would become entangled with those of another spider, lanced out at the same moment; both spiders would immediately seem to know the cause of the trouble, for as soon as their lines fouled they would rush angrily towards each other, each trying to drive the other from the elevation. Notwithstanding these difficulties, numbers were continually floating off on the breeze which blew from the south.
"I noticed three distinct species: one with a round scarlet body; another, velvet black, with large square cephalothorax and small pointed abdomen; the third and most abundant kind were of different shades of olive green, and varied greatly in size, the largest being fully a quarter of an inch in length. Apparently these spiders had been driven up from the low ground along the stream where it was wet, and had congregated along the borders of the dry ground in readiness to migrate.
"25th. Went again to visit the spiders, scarcely expecting to find them, as, since first seeing them, we have had much wind and rain. To my surprise
188 The Naturalist in La Plata.
I found them in greatly increased numbers: on the tops of cardoons, posts, and other elevated situations they were literally lying together in heaps. Most of them were large and of the olive-coloured species; their size had probably prevented them from getting away earlier, but they were now floating off in great numbers, the weather being calm and tolerably dry. To-day I noticed a new species with a grey body, elegantly striped with black, and pink legs--a very pretty spider.
"26th. Went again to-day and found that the whole vast army of gossamers, with the exception of a few stragglers sitting on posts and dry stalks, had vanished. They had taken advantage of the short spell of fine weather we are now having, after an unusually wet and boisterous autumn, to make their escape."
Here it seemed to me that a conjunction of circumstances--first, the unfavourable season preventing migration at the proper time, and secondly, the strip of valley out of which the spiders had been driven to the higher ground till they were massed together--only served to make visible and evident that a vast annual migration takes place which we have only to look closely for to discover.