Soil Sidekicks – Trapdoor Spider

The way our next Soil Sidekick hunts may seem like something out of a horror movie, but don’t worry – unless you are a pint-sized arthropod, you have nothing to fear from the trapdoor spider.

The elusive trapdoor spider measures in at a little over 1” in length. These smaller relatives of the tarantula are black or brown with a glossy exterior. Like other arachnids, they have eight legs and two body segments. They have relatively long lifespans, and can live up to 20 years.

Cork-lid trapdoor spider (Ctenizidae: Ummidia). Photo courtesy of Greg Gilbert from Dahlonega Area, Georgia, USA, via Wikimedia Commons.

Like the tarantula, the trapdoor spider is a mygalomorph, a member of a smaller group of spiders characterized by downwards-oriented fangs. Trapdoor spiders can be found all over the world, including species in the genus Ummidia and one known species in the genus Cyclocosmia in North Carolina. Although they are elusive, trapdoor spiders are considered common and can be found in your own backyard. Due to their elusive nature, these masters of camouflage are not well studied in this area.

The trapdoor spider is venomous like most spiders, but its venom is comparable to a wasp sting, causing only minor pain and swelling. These arachnids are not aggressive and will only bite people in self-defense. If you find one in your home, it’s best to capture it in a container such as a jar and release it outdoors.

Spiders have many interesting adaptations for survival in a world that is often harsh to such small creatures. Arachnids and their other cousins in the subphylum Chelicerata, including horseshoe crabs and sea spiders, possess specialized jaws called ‘chelicerae’. In trapdoor spiders, these mouthparts are modified with a row of hardened digging spines on each chelicera called a ‘rastellum’. These tiny projections resemble teeth or hairs, and are designed to help move the soil as the spider constructs its underground burrow.

Cork-lid Trapdoor spider burrow saved intact in baking powder tin ca. late 19th century. Photo courtesy of Jon Richfield, via Wikimedia Commons.

Although they may resemble a fifth pair of legs, enlarged mouthparts called ‘pedipalps’ are specialized in all spiders, like the chelicerae. These parts are used for shaping their web silk and assisting them in feeding, but also for reproduction in males.

Trapdoor spiders are named after the special burrows that they construct. They create their homes by digging down into the soil, lining them with silk that keeps in moisture and provides a surface on which the spiders can more easily walk around. The ‘trapdoor’ entrance is created using a blend of silk, soil, and other materials such as moss or leaves found around the burrow. This recipe makes a door that blends in with the environment, camouflaging the spider’s home. It is hinged with stretchy silk, and its underside has holes so that the spider can grip with its legs or fangs and spring out at just the right moment. This also allows the occupant to hold the door tightly closed if a predator attempts to enter the burrow.

Trapdoor spiders lay silk ‘tripwires’ around the entrance of their dens, then lie in wait for unsuspecting prey to come along and accidentally touch the silk. They can feel the vibrations along the silk, judging the size and distance of their (typically arthropod) prey based on what they sense. Once the spider senses that there is a meal waiting outside, it springs open the door, grabs its prey, and retreats once again into the burrow.

Cyclocosmia sp. in burrow, from Appalachia. Photo courtesy of Marshal Hedin, via Wikimedia Commons.

Ravine trapdoor spiders (Cyclocosmia truncata) have an interesting body shape. Unlike other trapdoor spiders, their abdomens are cut short to form a hard, patterned disc-like shape that serves as a stopper for their burrows. They use the width of their abdomens to seal themselves safely inside their dens, using their hardened backsides as a protective shield.

Certain species of native mud dauber wasps prey on any spider they can catch, including trapdoor spiders. A female wasp will paralyze the arachnid, bringing it back to the mud nest and laying her eggs in it while it is still alive so that her larvae have a fresh source of food when they hatch. The trapdoor spider’s other natural enemies may include mice or other spiders.

Although they can live longer, males often only live about nine months past maturity due to the fact that they leave their burrows and wander the landscape to find a mate in the springtime, placing them in much more danger than females.

Do female trapdoor spiders really eat males? That depends – sometimes a female spider will mistake a male for prey at her front door, so he must approach cautiously. Male trapdoor spiders have a special tooth-like hook on their first pair of legs used for hooking the female’s fangs to hold them back as he enters the burrow. If the female is not receptive to his advances, she could still decide to kill him, and he might become her next meal – however, this is not as common as you may think.

Although these interesting arachnids may be widespread, they are not often seen. Consider yourself lucky if you have the chance to see a trapdoor spider springing into action.

Soil Sidekicks is an inside scoop starring animals that live in and around the soil. Did you know that there are more living creatures in a shovelful of rich soil than there are people on the planet? Get the dirt on where the Sidekicks live and discover more about the soil that sustains us at Dig It! The Secrets of Soil.

Claire Carrington is a Museum Public Relations intern, and currently a senior at Campbell University.

Thanks to Dr. Colin Brammer, Coordinator of the Natural World Investigate Lab for information and guidance.

First photo: By Greg Gilbert from Dahlonega Area, Georgia, USA (Cork-lid trapdoor spider (Ctenizidae: Ummidia))
{CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)}, via Wikimedia Commons

Second photo: By Marshal Hedin (http://www.bio.sdsu.edu/pub/spiders/App/index.html)
{CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)}, via Wikimedia Commons

Third photo: By Jon Richfield (Own work)
{CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)}, via Wikimedia Commons