In the Cribellatae the metatarsus is always furnished with a comb-like organ, the “calamistrum,” correlated with an extra spinning apparatus, the “cribellum,” but this will be dealt with when we reach the systematic portion of the subject.
The general direction taken by the legs, the comparative length of the different joints, their armature of hairs, bristles, and spines, and the number and conformation of the tarsal claws, are points of great importance in the classification of Spiders.
Under considerable magnification the legs of all Spiders exhibit a number of minute organs, arranged with absolute uniformity throughout the Araneae, and known as the “lyriform organs.” They consist of little parallel ridges of thickened chitin, the slit between them being covered by thinner chitin. They are eleven on each leg, and are distributed near the distal extremities of each of the first six joints. Their function is unknown, though some authors consider them to be organs of hearing.
Fig. [181].—Spinnerets of Epeira diademata. A, Ventral view of Epeira; B, spinnerets magnified; C, profile.
The Spinnerets are normally six in number, and, except in rare instances, are placed beneath the abdomen, near its apex and immediately in front of the anal tubercle. Their arrangement varies greatly, but they can generally be recognised as comprising three pairs, a posterior (or superior) pair, a median pair, and an anterior (or inferior) pair.
In nearly all the Theraphosae the anterior pair are absent, while the posterior spinnerets are largely developed. In the Palpimanidae only the anterior spinnerets are present. When all six are found, the usual arrangement is in the form of a rosette, the median spinnerets being hidden by the others in repose, but this disposition is widely departed from. In Hahnia (Agelenidae), for instance, they are ranged in a transverse row at the end of the abdomen, the posterior spinnerets occupying the extremities of the row, and the median ones the centre.
These spinnerets are highly mobile appendages, and additional play is given to their action by the presence of articulations, much resembling the “false” joints sometimes found on the legs, on the posterior and anterior pairs. They are always at least bi-articulate, and sometimes present three or four joints. They are movable turrets on which are mounted the “fusulae” or projections where the tubes from the spinning glands open. These are often very numerous, especially in the orb-weaving spiders, where the spinning powers are most highly developed. They consist of two portions, a cylindrical or conical basal part, succeeded by a very fine, generally tapering tube.
In some spiders the fusulae are all much alike, but usually a few very much larger than the rest are noticeable under the microscope, and these are often alluded to as “spigots.” The smaller ones are also divisible into two kinds, a few short conical fusulae being noticeable amongst the much more numerous cylindrical tubes. We shall treat of the functions of the various fusulae later (see pp. [335] and 349).
Simon remarks that though the battery of fusulae is most complicated in those spiders which possess the greatest spinning powers, it is by no means among them that extremely long spinnerets are developed. The posterior spinnerets of some of the Hersiliidae are of great length, but these spiders spin very little except in forming their egg-cocoons.