The female land hermit crab has two openings on the under side of her third set of walking legs. If you have a crab that is willing to extend out of its shell far enough, you may be able to see the gonorpores. They are very small pinhole openings. Males do not have the openings. Female land hermit crabs also can have an egg sac on the side of their abdomen. This is hard to see also and can be confused with the molt sac.
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Male hermit crab:
Additional images and information on sexing your crab is found on Mrs Poppy Puff’s Site Please visit her site to help you determine if your crab is a boy or a girl.