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.
The under surface of the cephalothorax is protected by the “sternum” or “plastron,” a large plate of variable shape, usually notched at either side for the reception of the legs, and having in front a small plate, generally hinged, but sometimes soldered to it, known as the “labium.” This has no homology with the labium of insects, but is a true sternite, more correctly described as “pars labialis sterni.”
The labium and the maxillary lobes of the palpi more or less conceal the under surface of the caput. The shape of the sternum and of the labium, and the contour and degree of inclination towards one another of the maxillae, are important considerations in the taxonomy of Spiders.
Fig. [174].—Diagrammatic ventral view of a Spider. Cephalothorax—l, Labium; m, maxilla; p, paturon of chelicera; st, sternum; u, unguis of chelicera. Abdomen—a.t, Anal tubercle; c, colulus; ep, epigyne; s, stigma; sp, spinnerets; tr, tracheal opening.
The appendages of the cephalothorax, which are the chelicerae or jaws, the pedipalpi or feelers, and the four pairs of ambulatory legs, will be treated separately.
Pedicle.—The chitinous investment of the narrow stalk which unites the thorax with the abdomen is for the most part thin and flexible, with only slight indurations of various patterns on the dorsal surface, where it is in most cases more or less protected by the forwardly-projecting abdomen. Beneath, it is usually quite membranous, guarded only by a sort of collar formed by the raised border of the anterior portion of the abdomen at the point of insertion. In some Spiders, however (Dysderidae), there is a posterior sternal plate, the “plagula,” closely corresponding with the labium in front, which partly embraces the pedicle. In Hermippus (Zodariidae) the plagula is detached from the sternum, and is succeeded posteriorly by two smaller paired plates.
Abdomen.—The abdomen differs remarkably in shape in the different groups of Spiders. In some families the prevailing shape is more or less globular, and in others cylindrical, while it may be diversified to almost any extent by prominences or spines. Ordinarily no sign of segmentation is observable, but in Liphistius it is covered dorsally by seven well-marked chitinous plates.
In most Spiders the integument of the abdomen is uniformly soft and flexible all over, but it is not rare to find portions of it thickened and hardened to form “scuta.” In the Gasteracanthinae and the Phoroncidinae there is a great dorsal scutum armed with spines, while in several families there are species characterised by the possession of a smooth dorsal scutum; and in some a ventral scutum is present.