STRIDULATING ORGANS 



327 



spiders possessing them certainly do not seem to form a 

 natural group. 



Stridulating Organs. — When Arthropod animals are capable 

 of producing a sound, tlie 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 he inaudible to human ears. 



Westring ^ was the first to discover (1843) a stridulating 

 organ in the small Theridiid spider Asagena ijlialerata. The 

 abdomen, where the pedicle 

 enters it, gives off a chitinous 

 collar, ^vhich 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 lieen since found 

 in various allied spiders, of 

 which the commonest Eng- 

 lish species is Steatoda Mpunctata. In this group it is generally 

 possessed by the male alone, being merely rudimentary, if present 

 at all, in the female. 



In 1880 Campbell" observed that in some of the Theridiid 

 Spiders of the genus Leplitliyijhantes, the outer surface of the 



^ Nat. Hist. Tidsskr. iv., 18-13, p. 349. 

 " J. Linn. Soc. xv., 1S81, p. 155. 



Fig. 183. — Stridulating apparatus of Steatoda 

 bipunctata, i . Much enlarged. A, Ridged 

 and toothed abdominal socket ; B, striated 

 area on the cephalothorax ; C, profile of 

 the Spider, x 5. 



