Natural Wonders: Stories from the Natural History Collection

Tarantula Diorama

One of the many fascinating objects in our collection is a tarantula (Aphonopelma sp.) diorama. The diorama displays a tarantula exiting its underground burrow with a fresh molt just outside the entrance. On the left side of the scene, a different tarantula is caught in a deadly dance with a female tarantula hawk wasp (Pepsis sp.). The tarantula is North America’s largest spider measuring up to 11 inches in height (including legs). Despite its appearance, the tarantula (Aphonopelma sp.) has very small venom glands containing mild venom that is only strong enough to paralyze small insects. To humans, its bite would feel no more painful than a bee sting. 

Tarantulas are nocturnal and spend most of their lives underground in burrows. Like other spiders, tarantulas shed their external exoskeleton periodically in a process called molting. For young tarantulas, molting occurs several times a year as they grow. The frequency will decrease to roughly once a year or less once maturity is reached. The molting process even also allows these spiders to replace lost appendages and internal organs. 

At around 4 to 7 years old, a male tarantula will reach sexual maturity, It will shed its exoskeleton for the last time, develop tibial spurs on its front legs, and will leave its burrow in search of a mate. Locally, mating season takes place from late August through early October, and several males can have been spotted in Mt. Diablo around this time in search of a mate. Once a male has located a female, he will rhythmically tap his pedipalps at the entrance of her burrow similar to knocking on a door. As she approaches, he will then use his newly grown tibial spurs as hooks to clasp the female’s fangs (in order to avoid becoming dinner) and begin mating. Afterwards, the male will scurry off to continue looking for other females to mate with, growing weaker and finally dying as winter approaches. The female, however, will live for around 20 to 30 years. After mating, she will return to her burrow to lay roughly 100 to 150 eggs, of which about two will make it to adulthood.

Tarantulas play a crucial role in their ecosystem acting as biological controls of insect populations, and as a source of food for other creatures such as mammals, lizards, birds and wasps. Tarantulas typically lie motionless by their burrows when hunting, waiting for the opportune moment to grab their prey and deliver a venomous bite. The venom will paralyze their prey, while they secrete a digestive enzyme that liquifies their meal so they can suck their food up through their straw-like mouth opening.

Despite their hunting prowess, tarantulas can also be an easy meal for larger predators. A tarantula’s primary defense mechanism is the barbed hairs on the back of their abdomen. It can shoot these hairs in the eyes and mucous membranes of its tormenter in hopes of scurrying away while the predator is distracted. The most feared predator of a tarantula is the large black and orange female tarantula hawk wasp (Pepsis sp.). This imposing insect seeks out tarantulas during late summer. When she locates a tarantula, the wasp attacks and delivers a paralyzing sting underneath the spider’s leg. The wasp then drags the tarantula to a hole and lays a single egg inside the tarantula before covering it. Once the larva hatches it will begin to feed on the still living tarantula, avoiding vital organs, which keeps its host alive and fresh. 

However, predators like the tarantula hawk wasp are not the only scary threats to tarantulas—humans can be too! Human threats to tarantulas include cars, habitat destruction, and collection. It is illegal in many parks (including Mount Diablo) to collect tarantulas as pets. So, if you see one out and about, feel free to observe it but let it continue to play its important role in the local ecosystem.