106 SPIDERS [CH. 



at all events there are many creatures with what 

 appear to be very well developed stridulating organs 

 whose note has never yet been heard by any naturalist, 

 but there are doubtless numberless sounds beyond the 

 range of our hearing, which is limited, like the key- 

 board of a piano. 



Now such a stridulating apparatus has been de- 

 tected in many spiders, and always in one of three 

 situations either between the two parts of the body 

 (cephalothorax and abdomen) or between the palps 

 and the mandibles, or between the palps and the front 

 legs. In some of the Theridiidae the hind end of the 

 cephalothorax is roughened and fits into a sort of 

 socket in the abdomen which is provided with parallel 

 ridges, so that when the abdomen is vibrated the two 

 surfaces are rubbed together, but no one has yet 

 heard a sound produced by these spiders. The 

 stridulating Aivculariidae, however, are easily heard, 

 the sound in some cases being described as a kind of 

 whistle, in others it has been said to have the effect 

 of shot dropping upon a plate. 



There are two quite distinct purposes for which 

 sounds may be produced ; they may either serve as 

 a call from one sex to the other, or as a warning to 

 intruders. Obviously the first purpose requires a 

 sense of hearing in the sex appealed to, and it is 

 interesting to note that in the Theridiidae, which are 

 among the spiders which show some appreciation of 



