The spiny trapdoor spider <scp> <i>Blakistonia aurea</i> </scp> (Mygalomorphae: Idiopidae) as a model to better understand the natural history of cryptic spiders

Abstract The diverse infraorder Mygalomorphae includes fossorial and cryptic spiders, such as tarantulas and trapdoor spiders. Their ability to disperse is often highly limited, rendering some mygalomorphs as rare short‐range endemics. These species are vulnerable to landscape changes, often hard to detect, and have poorly understood reproductive biology, making it difficult to develop conservation management strategies. Mygalomorphs also frequently have high intraspecific male size variation, although it is unclear what explains this. The South Australian spiny trapdoor spider Blakistonia aurea Hogg, 1902 (Idiopidae) is a species with highly variable male sizes, a surprisingly widespread distribution and is well‐represented in museum collections. This species is also important for the conservation of the endangered pygmy bluetongue lizard ( Tiliqua adelaidensis Peters, 1863), which preferentially uses their burrows for shelter. We studied the biology of B. aurea as a model to investigate male size variation and the natural history of cryptic mygalomorph spiders of conservation significance. We test whether sexually selected male dimorphism or geographic variation explain the naturally high variation in B. aurea male sizes. We then describe burrow structure from two regions of South Australia, as well as aspects of the species' natural history, including breeding seasonality, egg sac structure, clutch size and interspecific interactions. Finally, we also report on a mating observed in laboratory conditions, providing the first detailed description of mating behaviour in an Australian spiny trapdoor spider (Idiopidae: Arbanitinae). We found significant differences in burrow lid size for mature females from different regions, but no evidence for male dimorphism or geography underlying the unusual variation in male sizes. The reproductive cycle of the species is very long, with mating occurring from autumn to early spring, egg sacs being produced in summer, and offspring remaining in the maternal burrow for several weeks to a few months. Overall, we provide novel insights that add to the existing body of Idiopidae natural history research, which will help better inform crucial conservation management.