62 Studies on Arthropoda. I. 



his escape, the low and broken tones of the rightful owner 

 gradually rise in loudness and shrillness and frequency until 

 they become a continuous low-pitched whirr, or high-pitched 

 growl, the burrow acting as a resonator". In a paper published 

 in 1900 the same author says: "One almost certain use of the 

 stridulating-organ is to give warning to intending trespassers, 

 of its own species". 



On 0. ceratophthalma Anderson wrote (Journ. Asiatic Soc. 

 Beng. vol. LXIII, 1894, p. 138) : "A loud croaking noise .... At 

 first I imagined it must be caused by frogs, so perfectly did it 

 resemble the croaking of these animals .... it proceeded from 

 the burrows of the Ocypoda crab which here fringed the beach 

 at high-water mark. The burrows are frequently, in coral sand, 

 very wide at their mouth (6 to 8 ins.) and then taper gradually 

 downwards, so that they act as excellent resonators. The cause 

 of the stridulation of the crabs was by no means apparent, the 

 animals were all lying hidden in their burrows, and several 

 were croaking at the same time, as if in concert". — Ortmann 

 visited a place inhabited b}' the same species; he says (1. c.) 

 that all specimens were in their burrows, and the sound produced 

 was "a deep bass-tone". 



It is easily seen that the curious structure of the fiddle in 

 0. macrocera and O. ceratophthalma is not explained by the 

 obser\'ations quoted. 



IV. On Stridulation. 



Among the above-named stridulating Decapoda the species 

 belonging to the family Palinurida? and the genera Pcnceopsis 

 and Acanthocarpus are purely marine animals. Ocypoda is ter- 

 restrial. Ovalipes ocellatus is amphibious, "not unfrequently taken 

 at the surface of the sea", and "perfectly at home among the 

 loose sands at low-water mark, and also abundant on sandy 

 bottoms off shore" ; "when living at low- water mark on the sand 



