The foot of the Jumping Spider is extremely interesting, and shows a very ingenious arrangement, by means of which the animal can run with difficulty on smooth polished upright surfaces, or retain its footing when it alights on such surfaces after a spring. We can see this arrangement in Fig. 56. Just behind the claws is a bundle of coarse hairs, technically called a _scop´ula_, or little brush. From these hairs adhesive matter flows, and in this fashion the spider literally sticks on. With this brush of hairs may be compared the tarsal cushions of many flies, and the adhesive hairs of Dytiscus and other beetles (see Chap. II)--though these last have a different function. Diving Spiders (_Argyrone´ta aquat´ica_) are especially suitable for our purpose. They are very common in most ponds, and in some places are so abundant as to be almost a nuisance to the collector who is in search of other things. Three of them are represented in Fig. 57. One is swimming; another is just entering the bell-shaped web under water; and the third is just climbing out of the water on to the broad floating leaves of the water crowfoot. [Illustration: FIG. 57.--Diving Spiders.] De Geer’s account of these spiders is extremely interesting[34], and we may verify it for ourselves, for these creatures may be kept without any trouble. They certainly agree, when a number are kept in captivity, much better than do other spiders. De Geer kept several in the same aquarium, and says that when they met they felt each other with their palps, and opened their falces, but he never saw them do any harm to each other. I have kept them under similar conditions with the same result. He tells us that these spiders spin under water a cell of strong, close, white silk, the shape of which he compares to a skull-cap, the half of a pigeon’s egg, or a diving-bell. In September, 1736, he first became acquainted with these creatures, and kept one in an aquarium for four months. It made its cell against the side of the aquarium, and the top of it rose above the surface of the water. (This was due to its being inflated. The web was not spun above the surface.) The walls of the cell were very thin, but it was filled with air, and the spider was resting inside, head uppermost, with its legs pressed against the body. About three months later he found that the mouth of the cell was closed, and the spider was comfortably settled in its winter quarters. When pressure was applied the cell burst and the air escaped, mounting up to the top in bubbles. The spider made its way out, and took an Asellus that was offered, and made a meal of it.