In this chapter we shall take examples from two Classes—the Arachnoi´dea and the Myriap´oda. To the first-named Class belong also the Scorpions, which, with the Book Scorpions, the Harvestmen, and some others, may be neglected here. This will leave us only the Spiders and Mites to deal with.
Every one knows a Spider when he sees one, though not every one can give offhand a definition that shall include the whole Order. Let us endeavour to express their characters in simple terms, keeping in mind the definition of head, thorax, and abdomen in Chapter II. In Spiders the head and thorax are joined together in one unsegmented portion, called the cephalothorax, and this is connected with the abdomen, which is also unsegmented, by a more or less slender stalk or peduncle. So that while Insects have three regions, Spiders have but two. We may express the difference thus:
| Insects | head, thorax | abdomen. |
| Spider | cephalothorax | abdomen. |
The cephalothorax bears six pairs of appendages (Fig. 48A). Taking these in order, there are—
1. A pair of falces (an), representing the antennae of insects. These have a movable claw-like joint at the extremity, perforated so as to convey into a wound the secretion from the poison-glands.
2. A pair of five-jointed pedipalps (p), sometimes called maxillary palpi, from the fact that the parts whence they spring correspond to the maxillae of insects (Fig. 21A, d). In the female the last joint terminates in a claw; in the male this joint is specially modified for sexual purposes (Fig. 48B).
3. Four pairs of walking legs, of which only the coxae (c) are shown in the diagram. The two front legs are often used as organs of touch.
Breathing is carried on by chambered air-tubes or lung-sacs, and in all our British Spiders (with the single exception of the native Trap-door Spider, which, by the way, does not make a trap-door) there are also air-tubes resembling those of Insects. The lung-sacs open to the external air by stigmata (st) on the abdomen, while the ordinary air-tubes open near the spinnerets (sp), the organs employed in the production of the silky threads from which are formed webs, nests, egg-bags, and ropes. By means of these threads, spiders drop from their webs to the ground, construct flying bridges from one point to another, and even in some sort travel through the air.
Spiders live upon the juices of their prey, which are drawn up into the stomach by means of a suctorial apparatus at the end of the gullet. The young, when they leave the egg, resemble their parents in all respects but size—that is, they undergo no metamorphosis.