Birgus

Birgus is a monotypic genus of decapod crustacean in the family Coenobitidae, containing the single species Birgus latro, the coconut crab, the largest living terrestrial arthropod, with a weight of up to 4.1 kilograms and a leg span exceeding 0.91 meters. Unlike its close relatives in the genus Coenobita, adult coconut crabs abandon the gastropod shells hermit crabs typically rely on, instead hardening their own abdomens with chitin and calcium carbonate. They breathe through specialized branchiostegal lungs rather than functioning gills, have evolved insect-like olfactory organs on their antennae through convergent evolution, and are so fully terrestrial that adults can drown if kept underwater. Birgus represents an evolutionary endpoint among the hermit crabs, having lost the shell-carrying habit that constrains its sister genus.