Stridulation iu Crustacea Decapoda. I. 6l 



Indo-Australian 0. ceratophthalma Pall, the area differs some- 

 what from that in 0. macroccra ; the area tapers upwards from 

 a little below the middle, and nearly these three-fifths have 

 somewhat coarse ridges moderatel}- remote from each other, 

 while further below the ridges are suddenly ver\' fine, all 

 equally fine and close together. The area can therefore 

 be compared with a fiddle bearing two chords; 

 when its major upper part is moved against the 

 ridge on ischium the tone produced is deep; when 

 only its lower part is used the tone is very much 

 higher. — 0. macroccra can even produce three tones in using 

 respectively the upper, the middle or the lower part of the area. 

 On these musical possibilities see later on. 



The species of the genus Ocypoda are not marine, but un- 

 fortunately our knowledge of their biology, especially of their 

 musical faculties, is imperfect. The best account of a species 

 is written by Alcock in "Administration Report of the marine 

 Survey of India for the official year 1891 — 92"; he observed 

 0. macrocera, and the part of his account (p. 16) necessary for 

 my purpose shall be reprinted here. "They are gregarious, and 

 though at times they ma}- be seen marching (migrating?) in 

 battalions across the sand, the}' usually live in "warrens" at and 

 above high- water mark, where they excavate tortuous burrows 

 .... and when alarmed flying each one to its burrow". "The 

 sounds can be heard, and their effect seen, by forcing one crab, 

 which we will call the intruder, into the burrow of another, 

 which we will call the rightful owner. The intruder shows the 

 strongest reluctance to enter, and will take all the risks of open 

 flight, rather than do so, and, when forced in, he keeps as near 

 the mouth of the borrow as possible. When the rightful owner 

 discovers the intruder he utters a few broken tones of remon- 

 strance, on hearing which the intruder, if permitted, will at 

 once leave the burrow. If the intruder be prevented from making 



