Why do Cyclocosmia spiders have an abdominal disk?

Cyclocosmia is a genus of mygalomorph trapdoor spiders in the family Halonoproctidae, first described by Anton Ausserer in 1871.

C. ricketti females are 28 millimetres (1.1 in) long, with a disk diameter of 16 millimetres (0.63 in). Their burrows are 7 to 15 centimetres (2.8 to 5.9 in) deep, and only the bottom portion of the burrow is silk lined.

These are trapdoor spiders, whose species are distinguished from each other by the pattern of the abdominal disk, the number of hairs on its seam, and the shape of the spermathecae. They have abdomens that are abruptly truncated, ending in a hardened disc that is strengthened by a system of ribs and grooves. They use this to clog the entrance of their burrows when threatened. The disks have strong spines around the edge, and they each have four spinnerets just anterior to it.