458 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



After crawling feebly about for three or four days picking up 

 what food it can find, the young crab molts. After the molt the 

 second crab (fig. 33) shows only minor and relatively slight changes 

 in structure. The carapace is now about l-£ millimeters long and 

 nearly 2 broad. It is thus very nearly the same shape as in the 

 adult crab. The only other notable change in structure is the further 

 atrophy of the abdominal appendages. These have become quite 

 minute and are entirely absent on the sixth segment. Although the 

 changes in structure have been slight, the second crab is much more 

 active than the first and runs about on the beach with more assur- 

 ance and speed. It still remains near some protecting bit of beach 

 debris, however. After about four or five days a second molt 

 occurs. 



The third crab (fig. 34) is slightly over 2 millimeters broad across 

 the carapace. It now runs about freely enough and may even dig 

 its first burrow. This burrow is only an inch or two deep and is 

 just wide enough to admit the little crab, but it is sufficient to pro- 

 tect him from the incoming tide and the lively small enemies that 

 come in along with the tide. The only considerable difference in 

 structure is the complete atrophy of the megalops pleopods. These 

 are generally absent entirely but they may be present as extremely 

 minute buds that may only be seen under the high magnifications 

 of a microscope (500 diameters). 



After about a week the third molt occurs. The fourth crab is 

 about 3 millimeters broad across the carapace. A careful examina- 

 tion of the little crabs of this size would show that they are no longer 

 all exactly alike in their external structures. In about half of them 

 one of the pincers is slightly heavier and larger than the other. 

 (Figs. 35 and 36.) These are the male crabs. On the first and 

 second abdominal segments of the males there are now developed 

 minute buds that will develop into the sexual appendages of the 

 adult fiddler. The females show no differentiations of the pincers 

 and an examination of their abdomens shows appendages on the 

 second, third, fourth, and fifth segments although these are still very 

 minute. From this time on sexual differentiation becomes more and 

 more pronounced. 



When the young crab is about 4 millimeters or a sixth of an inch 

 across its carapace it has become quite distinctly differentiated as a 

 fiddler crab. The large pincer of the male has become considerably 

 larger than the feeding claw, although it is shorter and thicker in 

 proportion than the enormously elongated claw of the adult male. 

 The carapace shows the square box shape of the adult, and on the 

 area around the mouth-parts there are developed brushy hairs on 

 which the water that is driven out of the gill chamber may be 

 aerated before it is sucked in again. This makes the little crab 



