It’s no secret that spiders are master architects (just look to the expert web creations of orb weavers), but did you know that there are some spiders that take their construction skills underground? With nearly 52,000 described species [1], the order Araneae encompasses a diversity of behaviors, particularly those dealing with the use of silk. Trapdoor spiders have an interesting adaptation where they employ silk not to make webs, but to cover their subterranean homes. Trapdoor spiders are closely related to tarantulas, and both make burrows, but trapdoor spiders take their home design to another level by including a camouflaged flap at the entrance that acts as a “door” [2].
To build a burrow, a trapdoor spider begins excavating soil to create a hole, which it then lines with silk. This silk helps stabilize the burrow structure, as well as maintains humidity and regulates temperature. To construct the entrance, the spider carries soil from the bottom of the burrow to the rim and presses it down to make a circular opening. Finally, the spider continues this process of soil compression and silk application at the entrance, but working mostly at the anterior edge of the opening. The addition of soil to only one side of the entrance creates a hinge, and the soil flap becomes the trapdoor [3]!
Trapdoor-building behavior seems to have evolved multiple times across spiders [4]. While superficially similar, not all “doors” are exactly the same. They can be thin and somewhat flimsy, called “wafer-lid” trapdoors, or thick and sturdy, called “cork-lid” trapdoors.
If a spider is feeling fancy, they might spruce up the door by incorporating nearby plant material, as seen above. While impressive, this behavior probably has more to do with blending in than displaying their keen design skills.
Additionally, silk or plant material can sometimes be seen radiating from the burrow entrance to increase the spider’s sensory area. This comes in handy when it’s time to find something to eat [2].
Whereas most spiders use silk to catch prey or actively chase down a meal, trapdoor spiders don’t leave their homes at all. Instead, they stay in their burrow and wait for food to come to them. Think of it like a “trap-Door Dash” situation. The spider lies in its expertly camouflaged retreat with just the tips of its front legs protruding. They wait until they feel the vibrations of a reasonably sized prey item and lunge out, grabbing it with their fangs and quickly retreating to the safety of their burrow to consume the meal in peace [2].
So, if your main objectives in life are to eat and reproduce, and your food comes straight to your door, what reason would there be to ever leave your home? For female trapdoor spiders, there’s no reason at all! A female will actually stay in the same burrow for her entire life, just expanding the burrow’s diameter as she grows in size. She may even plug her burrow’s entrance with soil or silk while she’s still inside.
Conversely, males will eventually leave their burrows. Mature males wander in search of mature females willing to mate by following the scent of their pheromones. Once he has found a female’s burrow, a male will use his front legs to tap on her door to communicate that he is the same species and interested in mating. If she’s receptive, she will open the door and let him in [5].
These impressive animals can be found in almost every habitat type from coastal dunes to chaparral to desert. Trapdoor spiders are particularly diverse in the California Floristic Province biodiversity hotspot [6]. The next time you’re hiking, birdwatching, or having a beach day, be sure to keep an eye out for these masters of disguise.
Written by: Emma Jochim is a fourth-year PhD candidate in the Entomology program at UC Davis. When she’s not studying trapdoor spiders’ evolutionary relationships, she likes to explore more informal ways to appreciate her favorite arthropods. Photography, creating arachnid themed art, and sharing fun facts on her social media are some of the ways she likes to bridge the gap between scientists and the public.
References
[1] World Spider Catalog (2024). World Spider Catalog. Version 25.0. Natural History Museum Bern, online at http://wsc.nmbe.ch, accessed on Feb. 23, 2024. doi: 10.24436/2
[3] Coyle, F. A., & Icenogle, W. R. (1994). Natural History of the Californian Trapdoor Spider Genus Aliatypus (Araneae, Antrodiaetidae). The Journal of Arachnology, 22(3), 225–255. http://www.jstor.org/stable/3705426
[2] Foelix, R (2011). Biology of Spiders, 3rd ed. Oxford University Press, Oxford, UK.
[4] Opatova, V., Hamilton, C. A., Hedin, M., De Oca, L. M., Král, J., & Bond, J. E. (2020). Phylogenetic systematics and evolution of the spider infraorder Mygalomorphae using genomic scale data. Systematic Biology, 69(4), 671-707.
[5] Main, B. Y. (1985). Mygalomorphae. In Arachnida: Mygalomorphae, Araneomorphae, Pseudoscorpionida, Amblypygi, Palpigradi (pp. 1-8). Brill.[6] Bond, J. E. (2012). Phylogenetic treatment and taxonomic revision of the trapdoor spider genus Aptostichus Simon (Araneae, Mygalomorphae, Euctenizidae). ZooKeys, (252), 1.
[Edited by Jacob Johnson]
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