De Geer then came to the conclusion that these cells under water were constructed for the purpose of affording the spider a retreat when the water was frozen over, so that they could not come to the surface for a supply of air. He found, however, by observation, that they were also made in summer by both sexes. In a cell of this kind the female deposits her eggs--from eighty to a hundred in number, enclosed in a cocoon of white silk--and keeps guard over them, with her head defending the entrance to the cell. He succeeded in finding out the method by which the Diving Spider fills its cell with air. He noticed that when the creature was moving about in the water, its body was covered with a layer of air, and that this air was renewed from time to time when the animal came to the surface and raised its abdomen above the water. Loaded, so to speak, with air in this fashion, the spider descended, and entered the cell backwards, leaving an air-bubble. Having repeated this several times, at last she removed all the water from the cell, introducing in its place an equal quantity of air. It is very easy to watch the Diving Spider making its dwelling under water, and filling it with air. First of all the web is woven in a bell or thimble shape between the sprays of water-weed, or against the bottom or side of the aquarium. It is curious to notice how practices that must be necessary when the creature is at liberty are continued in captivity where they are useless. A web constructed in running water, or even in a pond or ditch, is liable to be swept away or to be emptied of air by a very slight current, so its owner has recourse to a system of guys and supporting threads, which are not required when the spider is safely housed in a small aquarium. Nevertheless, the guys are made. In an eight-ounce bottle I have now a male Diving Spider, which has lived there for about seven months. Its cell is made between the whorls of a spray of milfoil, and guy threads have been carried to no less than five whorls--two above and three below the opening. Now that it is filled with air, the cell gleams in the water like a great bubble of quicksilver. The air may be expelled by shaking or tilting the bottle, and if the web be not damaged the spider will generally refill it with air, though sometimes it prefers to make a new dwelling. Fig. 58 shows the cell of a Diving Spider; the white lines represent supporting threads attached to the water plants.