A small papilla, the “colulus” (Fig. [174], c), is often observable, projecting between the anterior spinnerets. The “anal tubercle” (Fig. [174], a.t), on which the vent is situated, terminates the abdomen, and is generally in close juxtaposition with the posterior spinnerets.
Appendages.—The cephalothoracic appendages are the chelicerae, the pedipalpi, and the four pairs of ambulatory legs. Those of the abdomen are the mammillae or spinnerets.
Chelicerae.—These are two-jointed appendages, articulated immediately below or in front of the clypeus. They are the “mandibles” of many authors, but there is good reason for believing that they are not homologous with the mandibles of Insects. There is little agreement, moreover, with regard to the names given to the two joints of which they consist. The term “falx,” often applied to the basal joint, is much more appropriate to the sickle-like distal joint. Base and fang are tolerably satisfactory, or we may avoid ambiguity by adopting the terms “paturon” and “unguis” suggested by Lyonnet.[[257]]
The paturon is a stout joint of more or less cylindrical or conical shape. The unguis (the “crochet” of Simon) is hook-like, and can generally be folded back upon the paturon, which often presents a groove for its reception. The Theraphosid spiders are distinguished from all others by the fact that the plane of action of the chelicerae is vertical and longitudinal. The paturon projects forward in a line parallel with the axis of the body, and its distal end can be raised or depressed, but not moved laterally; while the unguis in action has the point directed downwards, and, at rest, is applied to the under surface of the paturon.
In other spiders the patura hang more or less vertically, and while to some extent mobile in all directions, their principal motion is lateral, and the ungues have their points directed towards each other in action, and are applied to the inner surfaces of the patura in repose. The plane of action in this case is also more or less vertical, but transverse.
Fig. [177].—Front view of Textrix denticulata. × about 10. 1, Caput; 2, eyes; 3, paturon; and 4, unguis of chelicera.
The paturon is always extremely hard and strong. In Theraphosae of burrowing habits the distal end is furnished with a group of powerful teeth, the “rastellus.” The groove for the reception of the unguis is often guarded on one side or on both by rows of teeth, the arrangement of which is frequently an important specific character. The inner anterior border is also often furnished with a group of stiff hairs or bristles. This powerful joint is of use in crushing and expressing the fluids of insects pierced by the ungues.
The crescent-shaped unguis is tapering and smooth, except for the presence, on the posterior surface, of one or two feebly dentated ridges. Near its free extremity there is a small orifice leading to the poison reservoir and gland.
In the genus Pholcus (see p. [401]) the chelicerae may almost be regarded as chelate, the unguis being met by a spiny projection from the inner anterior border of the paturon.