Trapdoor spiders (superfamily Ctenizoidea, family Ctenizidae) are medium-sized mygalomorph spiders that construct burrows with a cork-like trapdoor made of soil, vegetation and silk. However, there are also unrelated spider families that are sometimes called trapdoor spiders, such as the Liphistiidae, Barychelidae, Cyrtaucheniidae and some Idiopidae and Nemesiidae. Some Conothele species do not build a burrow, but construct a silken tube with trap-door in bark crevices.
The trapdoor is difficult to see when it is closed because the plant and soil materials effectively camouflage it. The trapdoor is hinged on one side with silk. The spiders, which are usually nocturnal, typically wait for prey while holding onto the underside of the door with the claws on their tarsi. Prey is captured when insects, other arthropods, or small vertebrates venture too close to the half-open trapdoor at night. The spider detects the prey by vibrations and when it comes close enough, the spider leaps out of its burrow and captures it.
A hungry individual will wait halfway outside of its burrow for a meal. Male trapdoor spiders can overcome the females' aggressive reactions to their approach, but it is not known how. Females never travel far from their burrows, especially if they have an eggsac. During this time, the female will capture food and regurgitate it to feed her spiderlings. Enemies of the trapdoor spider include certain pompilid (spider) wasps, which seek out the burrows and manage to gain entrance. They sting the owner and lay their eggs (usually one per spider) on its body. When the egg hatches, the larva devours the spider alive.
Unlike other mygalomorph spiders, the Ctenizidae have a rastellum on the chelicera. Resembling "teeth" or "barbs" on each fang, this modification is used to dig and gather soil while constructing a burrow. They use their pedipalps and first legs to hold the trapdoor closed when disturbed.
The taxonomy of trapdoor spiders is currently not well understood in the United States and many species of the common genus Ummidia remain undescribed. Ummidia is distributed across the southern United States. Bothriocyrtum californicum is the common trapdoor spider of the Pacific Coast. The strange genus Cyclocosmia includes seven species, found in Florida, Georgia, North Carolina, Mexico to Guatemala, China and Thailand. The discontinuous distribution is indicative of a basal genus that was affected by continental drift. The spiders of this genus are unusual in having a mask-like hardened plate on the opisthosoma, which seems to act as a second door to exclude predators, like the spider wasps. There is a narrow part of the burrow of these spiders where the abdominal shield just barely fits. Cyclocosmia torreya builds burrows in moss banks along the Apalachicola River in Florida. Other genera of trapdoor spiders are found in other areas of the world. They actually may be more common than we may think because of their cryptic habits. They do tend to be localized in distribution and as such may be subject to extinction because of local habitat destruction.