=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. [Illustration: 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. [Illustration: FIG. 184.—_Chilobrachys stridulans_ in stridulating attitude. After Wood-Mason. Natural size. ] Meanwhile the noise produced by a large Theraphosid spider in Assam (_Chilobrachys stridulans_) had attracted attention, and its stridulating apparatus was described in 1875 by Wood-Mason.[262] The sound resembled that obtained by “drawing the back of a knife along the edge of a strong comb.” Subsequently certain Sicariid spiders of a genus confined to the southern hemisphere were heard to produce a sound like the buzzing of a bee by the agitation of their palps, and both sexes were found to possess a very perfect stridulating organ, consisting of a row of short teeth on the femur of the pedipalp, and a striated area on the paturon of the chelicera.