1896. HOW AND WHY SCORPIONS HISS. 19 



known as the chela, pincer, or palp (the palp-finger of Wood-Mason), 

 which is used for seizing and holding prey, and is for this reason 

 loosely jointed to the body, and capable of extensive movements, up 

 and down and from right to left ; and since the hinder surface of its 

 basal segment is closely applied to the front or adjacent surface of the 

 corresponding segment of the first leg, it necessarily slides over it 

 when the pincer is in motion. In this spot, therefore, the conditions 

 for the production of a stridulating organ are most favourable, for, as 

 was explained in the article entitled " Musical Boxes in Spiders " 

 (Nat. Sci., vol. vi., p. 44, Jan. 1895), sound-producing organs in the 

 vast majority of Arthropoda are developed exclusively where friction 

 occurs between two adjacent chitinous areas. In addition, however, 

 to the great pincers, all scorpions possess a second pair of highly- 

 mobile appendages. These are the mandibles or chelicerae, which 



Fig. I. — Indian Scorpion (Scorpio swammerdami) Stridulating; two-thirds 

 natural size. 



have the form of small three-jointed pincers, lodged beneath the 

 front margin of the carapace or head-shield, and capable, like the 

 chelae, of considerable movement in all directions, and especially of 

 extension and retraction in a line with the long axis of the body. 

 When moved in this way the inner surface of the one can be rubbed 

 against the inner surface of the other, and the upper surface of both 

 against the anterior edge of the carapace. It is here that the new 

 compound organ to be described has been developed in the large 

 S. African scorpions of the genus Opisthophthalvms. 



Returning, however, for the moment to Wood-Mason's organ : As 

 has been explained, this exists between the basal segments of the 

 first leg and of the chela, and may easily be detected by the naked 

 eye when these two appendages are pulled apart from each other. 

 The keys or notes (the scraper of Wood- Mason) occupy the yellow 

 area on the coxa of the chela {see Fig. 2, A), and, as in the case of the 

 large so-called Mygale spiders, they are simply modified hairs, as may 

 be clearly seen by examining those situated close to the edge of 

 the area in question. Here the hairs are simply thickened and 



c 2 



