Idiosoma nigrum
Family Name: Idiopidae (Mygalomorphae)
These large, dark brown to black spiders (females may be 30 mm body length), are 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 but the sides are deeply grooved giving them a rugose (having many wrinkles or ridges), corrugated appearance. The burrows have a lightweight leaf-litter and silk trapdoor with a fan of leaf and twig trip-lines attached around the burrow rim.
These spiders are confined to south-western Australia in the dry woodlands and open forests east of the Darling Range and north to Moore River (Western Australia).
The Black Rugose Trapdoor Spider is well-adapted for life in semi-arid habitats. The burrows are 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 radiating out from the burrow entrance serve as a sort of remote sensing device - movement of a trip-line alerts the spider waiting in the burrow entrance to the presence of prey. Rather than foraging only at the burrow mouth (as many other trapdoor spiders do), these spiders charge out from under their doors to grab prey animals out on the trip-lines - especially ants, but also beetles, cockroaches, 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 Rugose Trapdoor Spider's thickened cuticle helps reduce water loss from the its body. However, a remarkable function of this 'armoured' abdomen is as 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 shield-like end of the abdomen - and there it sits (using part of the body to protect the rest of the animal is called phragmosis - Greek phragmos = a wall). Confronted by this seemingly impenetrable blockage, predators like scorpions, centipedes, hunting wasps and perhaps even birds, often give up and go away. But even this extraordinary defence has been breached by certain parasitic wasps. They can manoeuvre their paper-thin abdomens and long, slender egglaying tubes (ovipositors) down along the grooves on the spider's abdomen, allowing their eggs to be laid on the softer cuticle at the front of the abdomen. The wasp larva that emerges is difficult for the spider to dislodge from that area and it can safely start devouring its victim alive.
During autumn, the smaller, long-legged male spiders (up to 18 mm 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 rainstorms moisten and soften the ground. This ensures that the young dispersing on the ground are not going to suffer excessive water loss and will be able to dig their burrows.
This species, along with other trapdoor spiders, is threatened by land use activities, land clearance and habitat loss.
Copyright © Australian Museum, 2003
This page: http://www.amonline.net.au/factsheets/rugose.htm
