32 PROBLEMS OF RELATIVE GROWTH 



We can now proceed to check this deduction. Morgan 

 (1923A, 1924) has found that the very youngest post-larval 

 U. pugnax he could obtain have both claws alike, of female 

 or small type, in both sexes. In this stage, the chelae are 

 autotomized very readily ; and only when one is thus thrown 

 off does the other proceed to transform into a large chela. 

 After this has once happened, the fates of the two chelae are 

 irreversibly determined, though initially either may become a 

 large chela through the accident of the other's autotomy. 

 The moment of determination of the large chela appears 

 normally to take place very early, during the first or second 

 instar of post-larval life. 



Accordingly, I collected and weighed a number of the 

 smallest fiddler-crabs to be found on the beach, in which the 

 sexes could not be determined by casual inspection of either 

 chela-size or abdomen-shape. Their mean weight was about 

 6-7 mg. — an excellent approximation to the 5 mg. prophesied 

 on theoretical considerations. 



Then again, we can compare relative growth in different 

 species of the same genus. Uca minax is much scarcer near 

 Wood's Hole, and the comparatively few specimens available 

 were all of a size to be in the second phase of U. pugnax. 

 However, they yielded one or two interesting results. The 

 double logarithmic plot of chela against rest-of-body clearly 

 approximated to a straight line ; but owing to the smaller 

 number available it was impossible to determine the growth- 

 coefficient of the chela so accurately. It was, however, cer- 

 tainly between 1-58 and 1-66 — in other words, almost exactly 

 the same as that of U. pugnax for the first phase. Either 

 U. minax has no change in the growth-coefficient of the chela 

 at or near maturity, or at all periods its chelar growth-coeffi- 

 cient is higher than in pugnax. U. minax also differs from its 

 relative in the greater size which it attains ; the biggest 

 specimens found weighed 17-8 g. as against 3-6 g. for U. pugnax. 

 Correlated with this, as was to be expected, was the greater rela- 

 tive weight of the large chela to be found in minax. This in one 

 specimen amounted to no less than 77 per cent, of rest-of-body 

 weight, as against a maximum of 65 per cent, in U. pugnax. 



It is clear that the large chelae of big specimens of U. minax 

 must be getting close to their maximum limit of relative size. 

 A claw as big as the rest of the body would not be very prac- 

 ticable, and these are already over three-quarters this relative 

 size. In U. pugnax, where our figures are more accurate, 



