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Idiosoma nigrum (Family Idiopidae)

Shield-backed Trapdoor Spider, Black Rugose Trapdoor Spider

Distribution:

These spiders are confined to south-western Australia in the eucalypt-acacia dry woodlands and sclerophyll open forests east of the Darling Range and north to Moore River (WA).

Features: These dark brown to black spiders are large - females may be 30 mm in body length - and easily recognisable by the distinctive structure of the abdomen. The abdominal cuticle or "skin" is thick and hard. The end of the abdomen is flattened and shield-like, and its sides are deeply grooved giving them a rugose, corrugated appearance. The burrow always has a lightweight trapdoor of leaf-litter and silk, with a fan of leaf and twig trip-lines attached to around the burrow rim.

Ecology/Way of Life: The Black Rugose Trapdoor Spider is very well-adapted for life in semi-arid habitats. The burrow is deep enough (up to 32 cm) to ensure that air in the lower burrow remains humid and relatively cool in summer. The leaf and twig trip-lines that radiate out from the burrow's entrance serve as a sort of remote sensing device - movement of a trip-line alerts the waiting spider waiting in the entrance of the burrow to the presence of prey. Instead of foraging only at the burrow mouth, as many other trapdoor spiders do, these spiders charge out from under their light-weight door to grab prey animals on the trip-lines - especially ants, but also beetles, cockroaches, and millipedes and moths. This twig-line feeding strategy increases the spider's foraging area and its chances of catching food, especially when prey is scarce.

The Black Rugose Trapdoor Spider's thickened cuticle helps reduces the rate of water loss from the its body. However, the most remarkable function of this "armoured" abdomen provides a defence against predators. A few centimetres below its thin, wafer-like door, the vertical burrow narrows abruptly. When threatened by a predator the spider drops head-down into this narrowed section, plugging it perfectly with the end of its shield-like abdomen - and there it sits. Such use of part of the body to protect the rest of the animal is called phragmosis (after the Greek word phragmos = a wall). Confronted by this apparently impenetrable blockage, predators like scorpions, centipedes, hunting wasps and perhaps even birds, often give up and go away. But even this extraordinary defence can be breached by certain parasitic wasps. They can manoeuvre their paper-thin abdomen and long, slender egg-laying tube (the ovipositor) along the grooves on the spider's abdomen, allowing their eggs to be laid on the softer cuticle at the front of the abdomen. It is difficult for the spider to dislodge the wasp larva that emerges from that area, and the larva can safely start devouring its victim alive.

During autumn, the smaller, long-legged male spiders (up to 18 mm in body length) wander about looking for females. Mating takes place in the female's burrow. Eggs are laid during late spring and early summer and the young emerge from the egg sac in mid-summer. They remain in the mother's burrow until early winter, when rain storms moisten and soften the ground. This ensures that the young dispersing on the ground do not dry out too much, and will be able to dig their own burrow.

Interaction with Humans/Threats: This species, along with other trapdoor spiders, is threatened by land use activities, land clearance and habitat loss. No information is recorded on interaction with humans, or for possible threats to this species.

Other Comments:

Described by Barabara Main in 1952; the genus name comes from the Greek idios = personal or individual ("unique") and soma = body; the species name is from the Latin nigrum = black or dark, reflecting the colour of this spider.

Further Reading:

Main, B.Y. (1957). Biology of aganippine trapdoor spiders (Mygalomorphae: Ctenizidae). Australian Journal of Zoology 5 (4): 402-473.

Main, B.Y. (1976). Spiders. The Australian Naturalist Library: Collins, Sydney.

Main, B.Y. (1982). Adaptations to arid habitats by mygalomorph spiders. pp 273-283. In W.R. Barker and P.J.M. Greenslade (eds). Evolution of the Flora and Fauna of Arid Australia. Peacock Publishers, South Australia.

Topics: Spider webs

Acknowledgments:

Text & map by Mike Gray; photograph(s) courtesy Densey Cline

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