Palpal Organs.—Externally the essential parts of the palpal organ are three, the “haematodocha,” the “bulb,” and the “style.” The spines and projections, or “apophyses,” which often accompany the palpal organ proper, are of secondary importance, and in many spiders are entirely absent; nor is their function when present at all clear; but the infinite variety of design which they exhibit, and their singular uniformity in all the males of a species, render them of the utmost value as specific characteristics.
The “haematodocha” is the portion of the palpal organ attached to the tarsus, and often received into an excavation, the “alveolus,” on its under surface. It is a fibro-elastic bag, in its normal collapsed state usually somewhat spirally disposed round the base of the following portion, the “bulb.”
The bulb is generally the most conspicuous portion of the organ, and is a sub-globular sac with firm, though often semi-transparent, integument. Its base rests upon the haematodocha, and its apex is produced, often spirally, to a point which bears the seminal orifice. This external opening leads into a coiled tube within the bulb, ending in a blind sac, the “receptaculum seminis,” which projects into the haematodocha; and it is the aperture by which the sperm both enters and leaves the organ. How the sperm is conveyed to the receptaculum was long a matter for speculation, after the belief in a direct communication between the generative glands and the pedipalpi had been abandoned. The process has been actually observed in the case of a few spiders, which have been seen to deposit their sperm on a small web woven for the purpose, and then, inserting the styles of their palpal organs into the fluid, to suck it up into the receptacula seminis. This is probably the usual method of procedure, though it may be true, as some have asserted, that the palp is sometimes applied directly to the genital orifice.
The receptaculum and its tube being thus charged with sperm, it is the function of the haematodocha to eject it by exerting pressure on its base. For this purpose the haematodocha is in communication with the cavity of the tarsus, from which, in copulation, it receives a great flow of blood, and becomes greatly distended. Bertkau believes that he has detected very minute pores (meatus sanguinis) communicating between the haematodocha and the receptaculum, and allowing some of the blood-plasma from the former to mingle with the semen, but this appears to be very doubtful.
The Legs are uniformly eight in number, and are seven-jointed, the joints, counting from the body, being the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus.[[259]] In a few cases, through the presence of false articulations, i.e. rings of softer chitin, this number appears to be exceeded. Some of the Palpimanidae (see p. [398]) were at first thought to have only six joints on their anterior legs, but the tarsus is present, though very small.
In the case of most spiders, the legs take a general fore and aft direction, the first pair being directed forwards, the second forwards or laterally, and the third and fourth backwards. In the large group of “Crab-spiders” (Thomisidae), and in many of the Sparassinae, all the legs have a more or less lateral direction, and the spider moves with equal ease forwards, backwards, or sideways. The legs are usually more or less thickly clothed with hairs, but in some genera the clothing is so sparse that they appear glossy, while in others they have a positively shaggy appearance. Stouter hairs or “bristles” are often present, and some of the joints are also often furnished with “spines,” which in many cases are erectile.
The tarsi of all spiders are furnished with terminal claws, usually three in number, though in some families (Drassidae, Thomisidae, etc.) there are only two. The two principal claws are paired and usually dentated, though the number of their teeth may be unequal. The third claw, when present, is always smaller, median, and inferior.
Fig. [180].—Spider tarsi. 1, Tarsus of Epeira showing three claws and supplemental serrate hairs (a); 2, tarsus of a Thomisid Spider, with two claws; 3, 3a, lateral and dorsal view of tarsus of an Attid Spider, showing scopula at b.
In many spiders of climbing habits the place of the third claw is taken by a remarkable tuft of club-like hairs termed a “scopula” (Fig. [180], b), by means of which they are able to cling to smooth surfaces where claws would be able to obtain no hold. In some species there is a special false articulation—the “onychium”—at the end of the tarsus to bear the claws.