Idiosoma is a genus of trapdoor spiders in the family Idiopidae with three species, found only in Western Australia. The genus was transferred from Ctenizidae to Idiopidae in 1985. Idiosoma nigrum, commonly called the Black rugose trapdoor spider, can grow up to 30 mm long. Males reach a body size of up to 18 mm. The skin of their opisthosoma is hardened, with a flattened end and deep grooves running along the sides. The thickened skin helps to reduce water loss in its dry habitat. It also serves as a kind of plug to shield itself from predators. This phenomenon is called phragmosis and occurs in perfection in the spider genus Cyclocosmia (Ctenizidae). However, some parasitic wasps have evolved paper-thin abdomens and long, slender ovipositors and lay their eggs on the softer skin at the f