Feature

Trapdoor spiders mean business

Prof Ansie Dippenaar-Schoeman

Meet the trapdoor spiders, famed for their  silk-lined burrows complete with emergency exits,  hinged trapdoors and careful camouflage. Find out how the trapdoor spider gets down to the important business of living. 

Trapdoor spiders are a group of spiders that live permanently in silk-lined burrows (image below), which are closed with a well-fitting, hinged trapdoor of variable thickness. The trapdoor spiders are a diverse group of spiders found on the ground, under rocks or in trees. Southern Africa has a very rich fauna of trapdoor spiders represented by 5 families, 16 genera and about 217 species.

Why a burrow?

Burrows of the ground dwelling trapdoor spiders are frequently found in open grassy plains made vertically in the ground. A thick layer of silk covers the inside walls of the burrow. The silk-layer prevents the soil from caving in and helps to produce a well-balanced microclimate. It provides protection to the spider during inactive periods, especially during winter against thermal stress, as temperature and humidity stay relatively stable in the burrow. The eggs are usually deposited in the bottom of the burrow and are covered with a silk-layer to protect it from predators, parasites and microbial infections. The burrows provide protection during moulting and mating. The burrow entrances are used as a refuge to intercept or ambush prey. The waterproof silk protects the spider against flooding and the spiders are able to withdraw deep into the burrow during field fires.

Burrow construction

The spider normally digs only one hole during its lifetime and enlarges it, as they grow older. The depth of the burrows vary depending on obstacles in the ground, the size of the spider, hardness or softness of the substrate, soil type, and slope of the ground. Some burrow shapes (image below) found:
· single shaft of uniform or varying width.
· single shaft with a chamber at the bottom.
· shaft with side passages or shafts.
· shaft varying in shape e.g. Y-shaped or U-shape

 Various burrow shapes.

The trapdoor spiders use different excavating methods. They have special strong setae on their chelicerae, know as a rastellum, which is used to loosen the soil to initiate and excavate new burrows. During the initial excavation phase a burrow is made 1.5-2 times the spider's length. Then follows the door construction phase. During the third excavation phase the burrow is lengthened under cover of the trapdoor. Different methods are used to remove the soil from the burrow. The loose soil is ejected from the burrow by some species using their legs or palps. Other push the clumps of soil together with the front legs bound it together with silk threads and carry it in their chelicerae to the outside.

Burrow entrances

The thickness and shape of the trapdoors vary between families, genera and species.

Cork-lid trapdoors: One type of trapdoor resembles a cork of a bottle and is known as a cork-lid trapdoor (image at the top) which fits snugly into the burrow. The shape of the lid varies between species from round to D-shape. The ventral surface is usually unevenly convex with round edges and provided with a ring of small pits (image to the right). These pits provide a holding space for the spider to enable it to pull the lid close. The numerous strong setae on the front legs and palps are used in the process. When the spider closes the trapdoor it is very difficult to open.

Folding collars: The lining of the burrow sometimes extends slightly beyond the soil surface. This short, flexible collar collapse inward to close off the burrow and to camouflage the entrance. At night it is open when the spider takes position in the entrance. Debris is sometimes added to the extension as camouflage.

Pellet or stone lid: A pellet made of silk and soil particles or small stones are used to close the burrow entrance by some spiders.

Wafer-lid trapdoors: Consist of a flexible limp flap that is usually merely a continuation of the burrow's wall lining. The wafer-lid trapdoor usually stays open more easily while the spider is out, assisting with the rapid return to the burrow. In some species the thin wafer-lid is gradually transformed over a period of weeks from thin, flat and rather smooth-edged to ones that are thicker and more concave.

Trapdoor construction

The trapdoors are made of soil, often clay, modeled into shape and reinforced with silk. The outside of the lid is always well camouflaged and resembles the direct surroundings (see image to the right). Plant material found in the immediate surroundings is frequently incorporated into the door. Disguises used include bunches of short straws placed upright or across the lid or twigs stuck upright on the lid or pebbles.

Additional defense mechanisms

Although the burrows with the trapdoor provide protection against most natural enemies e.g. wasps and centipedes, some are still able to invade the burrows and additional defense mechanisms are used to overcome this problem.

Body plugs: the hardened posterior part of the abdomen in Galeosoma (Idiopidae) is used as a plug to provide a false bottom to close off parts of the burrow (image to the right).
Emergency exits: side shafts are provided with emergency exits through which the spiders can escape 
Folding collars: are build into sections lower down in the burrow and this is pulled closed to seal of the bottom part.
Side shafts: are closed by pebbles or trapdoors.
Pebbles and stones: pebbles and stones are positioned half way down the main burrow. When in danger the spider pulls the stone down to close the lower part off from the top part.

Prey-capture methods

Trapdoor spiders are sit-and-wait predators, except for the adult male who wanders around in search of a female. The prey is usually detected by substrate vibration receptors present on the palps and legs of the spiders.

The receptors could consist of trichobothria that detect prey-generated air currents or silk- or soil-vibration detectors such as the slit sensilla or club-shaped trichobothria. The prey usually consists of small animals such as insects, other arachnids, millipedes etc. that wander within range of the burrow and they cannot afford to be too selective. The spider lays in wait behind a slightly closed door for potential prey to pass. The spider then leaps upon it, flinging the door open in the process. They never really leave the burrow completely as the claws of the fourth leg keep a grip on the rim.

The hunting area is restricted to the direct surroundings of the burrow that could be reached by the spider. Spiders also make use of an early detective system. The spider increases the hunting detective area in using assembles of silk threads, twigs, grass or debris that they arrange around the burrows. The spider rest within the entrance with legs resting on the plant material or silk threads. Vibrations of prey movement are detected when they touch the material.

Trapdoor spider diversity

Members of the family Ctenizidae construct silk-lined burrows usually with rigid cork-like trapdoors that are either circular or D-shaped (seen at the top of the page). The Cyrtaucheniidae make single silk-lined burrows or burrows with side passages; frequently Y-shaped with flexible wafer trapdoors or they close the entrance with mud pellets. The Idiopidae live in silk-lined burrows or chambers closed with wafer- or cork-like trapdoors and the Nemesiidae  construct their borrows mainly in the soil surface either single or Y-shaped or silk-lined tunnels and chambers made under rocks. Some species of the Migidae  are found under bark where they construct a bag- or sac-like retreat. Small pieces of bark and moss are used to camouflage the burrow and the entrance.

Cyrtaucheniidae, the wafer-lid trapdoor spider

Idiopidae, the front-eyed trapdoor spider.

Nemesiidae, the wishbone trapdoor spider.



 Migidae, the tree trapdoor spider.

The spider guide of southern Africa

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