Though most spiders possess eight eyes, the number is sometimes smaller, and in some groups of eight-eyed spiders two of the eyes are sometimes so reduced and degenerate as to be practically rudimentary. As might be expected, Cave-spiders (e.g. Anthrobia mammouthia) may be entirely sightless.

Touch.—The sense of touch would appear to be extremely well developed in some spiders, and there is reason for believing that the Orb-weavers, at all events, depend far more upon it than upon that of sight.

Among the hairs which are distributed over the spider’s body and limbs, several different forms may be distinguished, and some of them are undoubtedly very delicate sense-organs of probably tactile function.

Spinning Glands.—Spiders vary greatly in their spinning powers. Some only use their silk for spinning a cocoon to protect their eggs, while others employ it to make snares and retreats, to bind up their prey, and to anchor themselves to spots to which they may wish to return, and whence they “drag at each remove a lengthening chain.”

All these functions are performed by the silk-glands of the Orb-weavers, and hence it is with them that the organs have attained their greatest perfection. We may conveniently take the case of the common large Garden-spider, Epeira diademata. The glands occupy the entire floor of the abdomen. They have been very thoroughly investigated by Apstein,[[266]] and may be divided into five kinds.

Fig. [187].—Spinning glands. A, Aciniform; B, tubuliform; C, piriform gland.

On either side of the abdomen there are two large “ampullaceal” glands debouching on “spigots,” one on the anterior, and one on the middle spinneret; there are three large “aggregate” glands which all terminate on spigots on the posterior spinneret; and three “tubuliform” glands, two of which have their orifices on the posterior, and one on the middle spinneret. Thus, in the entire abdomen there are sixteen large glands, terminating in the large fusulae known as spigots. In addition to this there are about 200 “piriform” glands whose openings are on the short conical fusulae of the posterior and anterior spinnerets, and about 400 “aciniform” glands which debouch, by cylindrical fusulae, on the middle and posterior spinnerets. Thus there are, in all, about 600 glands with their separate fusulae in the case of Epeira diademata.

The great number of orifices from which silk may be emitted has given rise to the widespread belief that, fine as the Spider’s line is, it is woven of hundreds of strands. This is an entire misconception, as we shall have occasion to show when we deal with the various spinning operations.

A few families are, as has already been stated, characterised by the possession of an extra spinning organ, the cribellum, and the orifices on this sieve-like plate lead to a large number of small glands, the “cribellum glands.”