Rostrum.—On examining a spider, even under a dissecting microscope, it will not be easy at first to discover the mouth. Indeed, Lyonnet had almost come to the conclusion that Spiders, like some Myrmelionid larvae, imbibed the juices of their prey by way of the mandibles, before he found the orifice and gave a remarkably accurate description of the adjacent parts.

If a specimen be placed on its back, and the labium raised while the chelicerae are pushed forward, no orifice is visible, but on careful examination it will be found that what appears to be a thick and fleshy labium is, in reality, two organs. The labium is thin and flat, and closely opposed to its upper surface is a somewhat flattened cone. This is the “rostrum,” and when it is separated from the labium the buccal orifice is disclosed. In a few spiders (Archeidae) in which the chelicerae are far removed from the mouth, the rostrum is tolerably conspicuous, but in most it is so hidden as to have escaped the observation of the great majority of observers. Schimkewitsch considers it homologous with the labrum of insects, but Simon thinks that it represents all the insect mouth-parts reduced to an exceedingly simple form. It is more probable that a beak consisting of a simple labrum and labium was a primitive Arachnid characteristic. If the rostrum be removed and its inner (or posterior) surface examined, a lance-shaped chitinous plate, the “palate,” becomes visible. It is furrowed down the middle by a narrow groove, which is converted into a tube for the passage of fluids when the rostrum is opposed to the labium.

Fig. [178].—Pedipalp of Tegenaria domestica ♂, × 5. 1, Coxa; 2, maxilla; 3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus; 8, palpal organ.

Pedipalpi.—The pedipalpi are extremely leg-like feelers, and are six-jointed, the metatarsal joint of the ambulatory legs being absent. The joints, therefore, are the coxa, trochanter, femur, patella, tibia, and tarsus (Fig. [178]).[[258]]

In the Theraphosae the coxa resembles that of the ambulatory leg, but in other spiders it is furnished, on the inner side, with a blade-like projection, the “maxilla” (Fig. [178]). The shape of the maxillae and the degree of their inclination towards the labium are of considerable taxonomic importance. The inner border of the maxilla is furnished with a tuft of hairs, which assist in retaining the juices expressed by the chelicerae, and its anterior border presents a cutting edge with a finely dentated ridge called the “serrula.”

In the female, and in the immature male, the remaining joints differ little from those of the legs, except that the tarsal joint is either clawless or has a single claw, which is generally smooth, and is never much dentated.

At the last moult but one the male pedipalp appears tumid at the end, and after the last moult the tarsus is seen to have developed a remarkable copulatory apparatus, the “palpal organ,” comparatively simple in some families, but in others presenting an extraordinary complexity of structure.

Fig. [179].—Diagram of palpal organ. 1, Tarsus; 2, bulb; 3, receptaculum seminis; 4, its aperture; 5, style; 6, haematodocha; 7, alveolus; 8, tibia.