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Art. V.—Notes on Spiders.
[Read before the Wellington Philosophical Society, 11th October, 1893]
1. How do the ordinary spiders which make geometric webs convey their lines across streams of water? 2. Do they see the opposite shore, or what is their extent of vision?
To obtain answers to the above questions; a potato, with a slice cut off it to give it a level base, was placed on the bottom of a large dish of water. Two sticks were stuck into the potato, about 8in. long, and 2in. apart at bottom, 4in. at top.
First a large spider was placed on one of the sticks. He ran up and down the stick, and then remained quiet.
Then a very small spider, was placed on the same stick. It ran straight at the large spider, which instantly grappled with it. The small spider escaped by dropping into the water (without any line attached, for which apparently it had no time). It was carried down to the potato by the force of its fall, and for more than a minute it travelled round and round the potato, under water; then it rose to the surface and lay still until blown to the stick, up which it ran nimbly, without having suffered apparently from its adventures under water or on its surface.
A medium-sized spider was then placed on the top of the stick. It dropped by a spun-out line to the surface of the water, felt it with its feet, and having thus ascertained, apparently, that it was imprisoned it climbed up its line, again. (the line seemed to be coiled up or carried up with it as it ascended). It then began to draw out line from the spinnerets with its legs, and to throw it out to leeward (there was a slight draught from the open window). The operation was very suggestive of throwing out a fishing-line. When about a foot of line had been thus thrown out floating in the air the creature ceased, and seemed to wait for it to catch on som thing. As there was nothing on which it could catch within 8ft. or 10ft. of it in the direction in which it was carried, by the draught of air, I brought a vase of flowers, on the table near enough to let the line touch some leaves, to which it seemed to adhere. Immediately the spider began to pull on the line, evidently to test if it was firmly attached, and it was just starting to climb out on to its bridge when unfortunately the large spider came blundering on to the scene, and ran up against the worker. There was a short conflict, and
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then they both ran away in opposite directions, the bridge having been broken in the mêlée. Shortly after the medium-sized spider had established a bridge between the two sticks, at about half their height. I presume, this was done in the same way as the first longer bridge; this one was about 2 ½in. long. He then went to the top of one stick, attached a line there, ran down, crossed the bridge, and carried the line to the top of the other stick, tightened it in and secured it—just as a sailor would have hauled taut and belayed. Then he crossed this upper bridge, strengthening it by an additional line which he spun out as he went. The large spider then crossed the upper bridge, spinning out his own line as he went. Then the two spiders started to cross this bridge from the opposite ends, met in the middle, sparred with one another, and both retired.
These observations seem to show that the geometric spiders do not see, or see very imperfectly, but that they have the instinct to throw out a line when they find-themselves on an isolated elevation; that they are very sensitive to draughts of air, and always lay out line to leeward. Apparently they trust to their good-fortune, when the line is floated out, that it may catch on something, and they are sensitive to the feeling of the line when it does catch. The hauling on the line to see if it is firmly enough attached to bear the creature's weight seems almost like reason, as does also the mode of forming a higher bridge by means of a lower one.
The fact that these spiders can walk on the bottom of a pool of water in case of need is remarkable, as it gives them another means of crossing narrow pieces of water. The spider could not walk on the smooth surface of the pan of water, but ran nimbly on the potato; so that a rough surface, to which it can cling, seems needful for the feat. Probably it carries a bubble of air down with it, and so can live for a time under water, and float up again before the air is exhausted.