ON THE YOUNG OF THE ROBBER CRAB'. 



By L. a. BORRADAILE, M.A., 



Lecturer of Selwyn College, Cambridge. 



In October 1874, when the 'Challenger' arrived at Zamboanga in the Philippines, 

 von Willemoes Suhm was anxious to investigate the development of Birgus latro from 

 the egg. Unfortunately for his intentions the breeding season of the animal is, in 

 that part of the world, the month of May. He made enquiries, however, on the subject, 

 and was told by an " intelligent native " that the young are born resembling the 

 parent. The robber crab being a land animal coming of a sea stock, this was not 

 a priori unlikely, though the small size of the female opening would not allow of a 

 very yolky egg. The suggestion has accordingly been accepted as likely by sundry 

 text-books. 



No further information on the subject has been forthcoming till the recent 

 publication by the present writer of a short note, showing that the young are hatched 

 in the zoaea stage (20). 



The facts of the case are as follows : 



The breeding season in the Philippines is the month of May, and in the Loyalty 

 Islands January and February. In Christmas Island it is also at the beginning of 

 the year. At this time the crabs, which ordinarily live more inland'^ may, in Lifu, be 



' Birgus latro (Linn.), 1767. (2, 15.) 



- The frequency of the visits of Birgus to the sea is still a matter of doubt. According to Darwin (8) 

 the crab is .said to go down to the shore every night "no doubt for the purpose of moistening its bratichiuf." 

 Von Suhm was told at Zamboanga that the creature "occasionally" visits the sea (17). Guppy (11) and 

 Whetham (10) also mention this habit. On the other hand Andrews (22) knows nothing of it, and Dr Willey 

 tells me that, out of the breeding season, Birgus is taken only inland and at night by torchlight, or can 

 be trapped in the same place by a cage baited with roasted coconut. The holes in whicli it lives are, at 

 Lifu, inland under coral limestone. At Zamboanga they are said to be .situated at the roots of trees in 

 swampy ground. It seems pretty certain that the principal food of the animal is the fruit of the Pandanus 

 and Coconut trees (6, 8, 14, 19, etc.) which it can obtain, if necessary, by climbing for it, and in this case 

 there can be little doubt that its object in visiting the sea out of the breeding season would be to moisten 

 its gills or lungs. If these be not moistened with fresh or salt water then we are met with two alternatives — 

 either tho interchange of gases must take place through a membrane which after a time becomes dry, or the 

 membrane must be kept moist by an exudation from the animal. Certain observations on tlie genus Cocnohita 

 lead the writer to suppose that the latter is probably the case. 



