108 



SPIDERS 



[CH. 



are vibrated. These keys are very curious structures. 

 They are of various lengths, and their shape will 

 perhaps be understood when it is said that a tolerable 

 model of one would be obtained by taking a flat iron 

 bar, sharpening it at the end, and then so twisting it 

 in the middle that the flat surface of one half is at 

 right-angles to the flat surface of the other half. Its 



Fig. 11. Three "keys" of a stridulating organ, after Spencer. 



appearance therefore varies according to the point ot 

 view, the narrow edge of one half and the broad edge 

 of the other being visible at the same time. A moment's 

 consideration will show that this torsion is calculated 

 to give great rigidity to the keys, for when the outer 

 half is struck on the flat surface the inner half opposes 

 its greatest diameter to the shock. A similar structure 

 is found in all the Theraphosid spiders which are able 



