SCORPIONS, SPIDERS AND CRABS 257 



burrow was occupied . . . and I think that this con- 

 sideration gives us a clue to the use of the stridulating 

 mechanism. At any rate, I was often able, after my 

 first accidental discovery, to elicit the sound, by catching 

 one of these crabs and forcing it into a burrow which 

 I knew was already occupied : the intruder would never 

 go far in, but would crouch just inside the mouth of the 

 burrow, and if it were made to travel deeper, then the 

 voice of the rightful owner would be hea5d in indignant 

 remonstrance from the depths." Another species, the 

 Grey Ocypode Crab {Ocyfoda ceratophthalmus), possesses 

 a similar instrument, and makes therewith a loud, croak- 

 ing noise. But it does not often burrow deeply. Colonel 

 Alcock therefore suggests that in this case it may be used 

 for scaring enemies. 



That these curious musical instruments may also be 

 used in mate-hunting seems highly probable. If the 

 stridulation is produced on one occasion to announce 

 the fact that callers are not desired, it may on another 

 signify an equally emphatic invitation to enter, the mood 

 of the occupant being expressed by the character of the 

 sounds emitted. It is significant, at any rate, that there 

 are no external sexual differences in these species; hence 

 the probability that it is by stridulation that the sexes 

 distinguish one another. 



This view seems to obtain confirmation from the fact 

 that the Crabs of the genus Gelasimus, or " Fiddler-crabs," 

 which are near relations of the ocypode Crabs, and, like 

 them, live in burrows in large companies, and are exposed 

 to the same enemies, which they avoid in the same way 

 by burrowing, have no stridulating mechanism, but 

 the sexes are strikingly different. This is especially so 

 in the case of the nippers, or chelipeds. These, in the 



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