Antrodiaetus pacificus – The Lurkers of the Pacific Northwest

Several days ago, James found an immature Antrodiaetus (a mygalomorph, similar to a tarantula with fangs that plunge parallel and downwards into prey. They are different from the true spiders (Araneomorphs) such as orb weavers, jumping spiders and wolf spiders, for example, as true spiders have fangs and mandibles that operate in a pinching movement. There are some other differences though!) There are around 21 species, and I know of three that live in Washington State – Antrodiaetus pacificus, which is what I suspect this immature female to be, only based on Arachnologist Rod Crawford’s ID of the last Antrodiaetus we found here on the property (could be another one, I wouldn’t be surprised that more than one species lives here, but again, I’m speculating!) Antrodiaetus pugnax, which I believe is what Rod identified my Renton, WA specimens as, and A. occultus, one I’d found for him during a collecting trip several years ago, near Mt. Rainier. I’ll have to do some research about the diversity within Washington State later, and update this post.

Female Antrodiaetus cf. pacificus

This one was running around outside of her burrow tonight. Females are almost never seen walking about, as they spend most of their lives in tunnels they dig into rotten logs or earthy substrates. We usually see males, but that is during their breeding season, when they roam about in search of receptive females.

Sometimes, when there are heavy rains their deep burrows become flooded. That means a search for a safer place to hide. They have two pairs of book lungs on the underside of their abdomen and can drown fairly quickly if these breathing organs become overly saturated. They require a particular micro-environment so that they do not dry out (which happens fairly easily with these guys, as their exoskeleton is quite primitive) but don’t do well where it is sopping wet. It isn’t raining today, though!

If you are live in Antrodiaetus territory (many species in the PNW) you can observe these spiders at night, when they lurk at the entrances of their burrows for passing prey. These burrows are round holes in the soil, lined with silk. (Without a burrow, they basically don’t eat – it is essential to their way of life.) They are very sensitive to changes in light, retreating if bright lights pass over them too strongly for too long, or suddenly.

In her silken retreat, a tube which she constructed in a pine needle, soil, moss and bark substrate. The tunnel system was several inches long, and was also chambered.

In her silken retreat, a tube which she constructed in a pine needle, soil, moss and bark substrate. The tunnel system was several inches long, and was also chambered.

If you live in Washington state, there is a good chance you have a bunch of them in your backyard.  In the daytime, these holes are perfectly hidden by a folding door, hence the common name – ‘folding trapdoor spider’. They are harmless, but can be a bit panicky when handled and do have fairly large fangs. They can’t climb glass or smooth surfaces and are awkward on such, having tarsal claws suited for a terrestrial life that focuses on digging. I’ve had a few for pets and have always marveled at the way they build their homes! You don’t see them much, unless you take up watching them at random times during the night.

They are preyed upon by spider hunting wasps who sting and paralyze them, drag them into a burrow, lay eggs on them and leave. While Antrodiaetus is still alive but immobilized, the larvae feast on her.


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