Fig. [182].—A, Spinnerets of Amaurobius similis ♀. Much enlarged. a, Anus; cr, cribellum; i.s, inferior spinneret; m.s, median spinneret; s.s, superior spinneret. B, Part of the 4th leg of the same Spider, showing the calamistrum (ca) on the metatarsus.
In addition to the six spinnerets, and just in front of them, there is to be found in some spiders an extra spinning organ in the form of a double sieve-like plate, the “cribellum.” This is always correlated with a comb of curved bristles on the metatarsi of the fourth pair of legs, the “calamistrum.” Such importance is assigned to these organs by Simon, that the Araneae Veraeare divided by him according to whether they are present or absent, into Cribellatae and Ecribellatae. This is probably an exaggerated view of the importance of these organs, and the spiders possessing them certainly do not seem to form a natural group.
Stridulating Organs.—When Arthropod animals are capable of producing a sound, the result is nearly always obtained by “stridulation,” that is, by the friction of two rough surfaces against each other. The surfaces which are modified for this purpose form what is called a “stridulating organ.” Such organs have been found in three very distinct Spider families, the Theridiidae, the Sicariidae, and the Aviculariidae. Hitherto they have only been observed in three positions—either between the thorax and abdomen, or between the chelicerae and the pedipalpi, or between the pedipalpi and the first legs.
In the Sicariidae and the Aviculariidae, the sounds have been distinctly heard and described. Those produced by the Theridiidae would appear to be inaudible to human ears.
Fig. [183].—Stridulating apparatus of Steatoda bipunctata, ♂. Much enlarged. A, Ridged and toothed abdominal socket; B, striated area on the cephalothorax; C, profile of the Spider, × 5.
Westring[[260]] was the first to discover (1843) a stridulating organ in the small Theridiid spider Asagena phalerata. The abdomen, where the pedicle enters it, gives off a chitinous collar, which projects over the cephalothorax, and has the inner surface of the dorsal part finely toothed. When the abdomen is raised and depressed, these teeth scrape against a number of fine striae on the back of the posterior part of the cephalothorax. A similar organ has been since found in various allied spiders, of which the commonest English species is Steatoda bipunctata. In this group it is generally possessed by the male alone, being merely rudimentary, if present at all, in the female.
In 1880 Campbell[[261]] observed that in some of the Theridiid Spiders of the genus Lephthyphantes, the outer surface of the chelicera and the inner surface of the femur of the pedipalp were finely striated at the point, where they were rubbed together when the palps were agitated, but though the appropriate motion was frequently given, he could hear no sound.
