The Araneae, even as at present known, form a very extensive and widely-distributed order of animals. Compared with certain insect orders, they have received little attention from the collector, and the number of known forms is certain to be very largely increased. They form an extremely compact and natural group, for though, within the order, there is an infinite variety of detail, their uniformity in essential points of structure is remarkable, and they are sharply marked off from the neighbouring groups of Arachnida.
Fig. [172].—Epeira angulata. ♀.
It is perhaps unfortunate that the obtrusiveness of particularly unattractive specimens of the race has always caused spiders to be regarded with more or less aversion. This prejudice can hardly fail to be modified by a wider acquaintance with these animals. There are certainly few groups which present points of greater interest in respect to their adaptation to special modes of life and the ingenuity displayed in the construction of their nests and the ensnaring of their prey.
Spiders are wingless, yet they may often be observed travelling through the air. They are air-breathing, yet many are amphibious in their habits, and one species at least spends the greater part of its existence beneath the surface of the water. On land they may be found in all imaginable localities which admit of the existence of that insect life on which they depend for food.
External Structure.—The spider’s body consists of two portions, the cephalothorax and the abdomen.
Cephalothorax.—Looked at dorsally (Fig. [173]), the cephalothorax is generally seen to have a depression near the middle, the “median fovea,” and from this certain lines, the “radial striae,” radiate towards the sides. These depressions indicate the attachment of internal muscles.
The head region or “caput” lies in front of the foremost of the radial striae, and is often clearly marked off from the thorax, and different from it in elevation. It bears the eyes, which, in the great majority of spiders, are eight in number. Many, however, are six-eyed, while in rare cases the number is reduced to four (Tetrablemma, see p. [404]), or even to two (Nops, see p. [395]). The number, relative size, and particular arrangement of these eyes are of considerable systematic importance. Their disposition varies very greatly, but it is generally possible to regard them as forming two transverse rows, an anterior and a posterior, each possessing a pair of median and a pair of lateral eyes.
Fig. [173].—Diagrammatic dorsal view of a Spider. ch, Chelicera; f, median fovea; n, normal marking; o, ocular area; p, pedipalp; st, stria. (The dotted line should reach the radial marking on the cephalothorax.)