Stridulation in Crustacea Decapoda. I. 57 



family Ocypodidse ; finally in some species of Sesarma and Hete- 

 rograpsus (Hilgendorf, de Man, Ortmann) of the family Grapsidae. 

 In most cases it is easy to recognize a stridulating organ. 

 Excepting in the genus Potamon and perhaps in Thalassina 

 (see later on) it consists always of a very regular row of granules 

 or small tubercles or a file-like series of ridges, playing the role 

 of a fiddle, while the role of the bow is performed by a ridge 

 or a small, regular row of granules or very short ridges, or by a 

 sharp margin. In some cases, as in Ocypoda, the fiddle is moved 

 to and fro against the ridge-shaped bow; in other cases, f. inst. 

 in PencBopsis, the sharp margin acting as bow is moved forwards 

 and backwards against a ridged area. The structure in Potamon 

 is completely different: a patch of modified spines on the coxa 

 of each of the two anterior pairs of thoracic legs is moved 

 against very modified spines on thickened lobes from the lower 

 margin of the carapace (Caiman, Ann. Mag. Nat. Hist., ser. 8, 

 vol. I, 1908, p. 469) ; the structure in Thalassina is unknown to 

 me. The sound has been heard in living animals of at least 

 two species of Palinurus (several observers), some species of 

 Ocypoda (Hilgendorf, Ortmann, Anderson, Alcock), Thalassina 

 anomala (Pearse) 1 ), and Matuta (Ortmann). But even if living 

 animals have not been observed, the organ is easily interpreted 

 when both the fiddle and the bow are well developed, or, as is 

 the case in Penceopsis, the fiddle is so characteristic that no 

 doubt remains. But in some cases, f. inst. in Coenobita rugosa, 

 the interpretation of the structures in question is less certain, 

 as either the fiddle or the bow is not developed in such a way, 

 that it is possible to decide with absolute certainty whether the 



x ) Pearse's treatise scarcely exists in Copenhagen. But according to Cai- 

 man's "record" the animal stridulates, and as I have been unable to dis- 

 cover the organ in question it may be supposed that it is either aber- 

 rant in structure or perhaps to be found in some unexpected or hidden 

 place, consequently that Pearse who studied the habits of the animal, 

 heard the stridulation. 



