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. [Illustration: 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.) ] In many spiders all the eyes have a dorsal aspect, but in some groups (Attidae, Lycosidae) the prevailing arrangement is to have the anterior eyes directed forwards and the posterior upwards. In other spiders, again, a dorsal view may only show the eyes in profile, all having their axes directed forwards or sideways, or they may be mounted on turrets, and thus command a wide range of view. The rows are described as straight, “procurved” (with the convexity backwards), or “recurved” (with the convexity forwards). Thus, in Fig. 177, the anterior row is slightly, and the posterior row considerably “recurved.” Sometimes there is a marked difference in the colour of the eyes, two or more being black, while the remainder are pearly white. In other cases they are homogeneous, either of the black or the white type. Simon considers the black eyes to be diurnal and the white nocturnal, but the evidence for this is indirect and not altogether satisfactory. The portion of the caput occupied by the eyes is often alluded to as the “ocular area.” The space between the ocular area and the chelicerae, well shown in Fig. 177, is known as the “clypeus.” It is usually more or less vertical, but in the Aviculariidae (see p. 386) it is horizontal and dorsal.