The abdomen is the third major part of the body of arthropods. In true crabs, the abdomen is normally wrapped under the body and pressed against the thorax (unlike in lobsters, for example, where the abdomen is the tail of the lobster). The width of the abdomen generally varies quite a bit between male and female fiddler crabs. In males, the abdomen is relatively narrow and rectangular and takes up only about a third of the ventral surface of the body. In females, the abdomen is substantially wider and round, taking up most of the ventral surface of the crab. The sex organs (gonopods and gonopores) are located on the hidden side of the abdomen, which can be moved away from the body during mating.
