The eyes of spiders are simple, and six or eight in number. These vary much in size and relative position, and furnish characters of great importance in classifying species. Those of the Hunting Spiders, which make no web, but pursue or spring upon their prey, are usually arranged in three rows; while those which make webs for the capture of prey have the eyes in a double row. In all cases they are on the front part of the upper surface of the cephalothorax.
Fig. 48.—A. Scheme of under surface of Wolf Spider (female). B. Pedipalp of male (enlarged). For other references, see text.
The Common Garden Spider (Epei´ra diade´ma) is a very good subject, and there can be no difficulty in procuring any number of specimens. The smallest garden will be sure to yield a plentiful supply; and even if we have no garden, a very superficial search among the hedgerows will give us as many as we can possibly want. Every one knows this spider, and the beautifully regular web which it makes. First of all, what one may call the outline of the web is spun—strong threads stretching from point to point, to which are attached lines radiating from a common centre. This may be represented diagrammatically by drawing a circle and producing radii from the centre to the circumference in all directions, or by making a rectangular figure and drawing lines to the boundaries from the point where the diagonals intersect. It must not, however, be supposed that the outline of the web is of a regular form. In this respect the spider adapts itself to circumstances, and spins a greater or less number of supporting threads, as may be necessary.
Fig. 49.—Garden Spider and Web.
Then the spiral is made (Fig. 49) from the centre to the circumference. The first few turns are of the same character as the radial lines; but all the rest of the short lines forming the spiral, and connecting the radial lines, are coated with a viscid secretion, which is the essential part of the snare; for the victims are really limed like birds on a twig, not entangled in the threads. It is well to test the character of the different parts of the spiral, not only by touch, which is convincing enough, but with pocket lens. Our low powers will not give such results as are shown in Fig. 50, but we shall have no difficulty in distinguishing the sticky parts from those not coated with the secretion.
Fig. 50.—A Adhesive Threads of Spiral. B. Non-adhesive Radial Threads.
It may be doubted whether all the Spiders of this genus make the spiral in the same way, for observers differ in their description of what they have seen. Some say that a non-adhesive spiral from centre to circumference is first made, and that the spider then moves ‘in a closer spiral from the circumference inwards, biting away the former spiral, replacing it by another, which is viscid and adhesive[29].’ Dr. Butler, on the other hand, who ‘watched spiders for months together, petting, feeding, and trying experiments with them every morning,’ after describing the making of the first and non-adhesive part of the spiral, says, ‘This line is not carried to the boundary, but at some distance from the centre a second is commenced, formed of extremely viscid silk, upon which the gummy secretion is distinctly visible, with the aid of a lens, in the form of closely approximated globules of amber-coloured glue. It is said that when the viscid lines are completed the spider cuts away the unadhesive lines, but this I have never observed[30].’ My own observations lead me to believe that Dr. Butler is correct in his description. Though I have often watched, I have never seen a spider ‘biting’ away any part of its web, nor would the falces appear to be adapted for such an operation. They are piercing, not cutting, weapons.