#ThreatenedSpecies of the Week: shield back spider

Idiosoma nigrum, or shield-back spider is listed as Vulnerable under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) and also Vulnerable under the Wildlife Conservation Act 1950.

The main threats to the spider are land clearance and habitat fragmentation, salinity, and grazing of habitat by stock and feral animals. Inappropriate fire regimes are also considered a potential threat to the species across its range.

The Shield-backed Trapdoor Spider is dark brown to black, large and with a distinctive thick and hard cuticle on the abdomen.
The Shield-backed Trapdoor Spider is dark brown to black, large and with a distinctive thick and hard cuticle on the abdomen.

The Shield-backed Trapdoor Spider is dark brown to black, large (females up to 30 mm in body length) and with a distinctive thick and hard cuticle on the abdomen. The end of the abdomen is flattened into a shield and the sides are deeply corrugated.

The Shield-backed Trapdoor Spider is endemic to semi-arid south-west Western Australia.

It occurs in a number of severely fragmented populations in the central and northern Wheatbelt. Further north, the species occurs in more arid areas in the Northern Agricultural Region (NAR) such as large isolated ranges at Jack Hills, Weld Range and Blue Hills and coastal areas of the NAR, for example, Zuytdorp Station north of the Murchison River.

The arid Northern Agricultural Region’s populations are naturally fragmented or isolated because they persist only on ranges, but the Wheatbelt and coastal NAR populations are all severely fragmented as a result of land clearing.

The species is very well-adapted for life in semi-arid habitats and lives in burrows that are tubular and approximately 20–30 cm deep. The burrow is deep enough to ensure that air in the lower burrow remains humid and relatively cool in summer. The burrow has a lightweight trapdoor of leaf-litter and silk, with a fan of leaf and twig trip-lines attached to the burrow rim.

Movement of a trip-line alerts the spider waiting in the entrance of the burrow to the presence of prey, which is primarily ants, but also includes beetles, cockroaches, millipedes and moths. This reliance on leaf litter means that leaf litter loss through inappropriate fire regimes and management may impact significantly on the species ability to feed.

Females spend their entire life in the burrow or within its proximity. Gene flow is therefore facilitated by male biased dispersal, estimated to be less than 500 m, as only males leave their burrows in search of females. The species becomes dormant during the drier months of the year from November to February.

More information on the Idiosoma nigrum (shield-back spider) can be found in this Government-approved Conservation Advice document.

Photo: Trapdoor Spider burrow by Vanessa Westcott from Bush Heritage Australia.

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