Bay Nature magazineFall 2018

Tarantulas Emerge to Wander the Landscape in the Autumn

October 1, 2018

Seven years or so after hatching, upon reaching sexual maturity, the male Aphonopelma iodius leaves his burrow to find a mate. He most often searches at dusk and dawn and sometimes travels more than three-quarters of a mile. Although he wanders during autumn at the same time as other males, this is not a true migration: Individual males of this species don’t move in synchrony or travel in the same direction.

tarantula waiting at burrow
This male Bay Area blond tarantula (the only tarantula species found in the Bay Area) traveled as far as three-quarters of a mile to the edge of this female’s burrow where he sits very still on an October morning in Pinnacles National Park. (Photo by Ben Witzke)

Aphonopelma iodius is often called the Bay Area blond tarantula because it is the only tarantula in the Bay Area. Females have pale hair on their legs and the front of their body; males are darker than females, with light brown, iridescent hair. The largest individuals have a 4-inch leg span.

Before beginning his journey, the male prepares for copulation. He’ll transfer his sperm to a female using his pedipalps, the paired leg-like appendages next to his fangs. His pedipalps contain sacs in which he’ll store seminal fluid, but they are too short to reach his genital opening. So in a process called sperm induction, the male spins a special web shaped like an inverted bowl into which he deposits sperm from his genital opening. He then perches above the web and spends a few hours drawing its contents into his pedipalpal sacs.

The journey he’s about to embark on is dangerous. A car might crush him as he crosses a road, or a coyote might eat him. One defense consists of sharp irritating hairs on his abdomen that, using his hind legs, he can flick into the face of predators. The hairs are barbed and, when embedded in human soft tissue, cause a bothersome itch. But they won’t protect him from his most dangerous adversary: a female wasp in the family Pompilidae, often called a tarantula hawk. She can subdue him with a sting, drag him to her burrow, and lay a single egg on his paralyzed body. Some days later, the larva would then hatch and consume the tarantula alive.

The female tarantulas’ burrows are scattered across the landscape, so the male employs a unique strategy to find them. He alternates between walking in straight lines and concentric circles (according to studies of related species), until he detects a female’s scent, suggesting that a burrow is nearby. The male signals his presence by tapping out a series of percussive rhythms on the ground. He first produces a rapid staccato with his legs, then drums an asynchronous counterpoint with his pedipalps; and, finally, he quivers his body to produce a high-frequency vibration. If the male performs to the female’s liking—she feels his drumming through the ground—she will emerge from her burrow.

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When they meet, the partners gently tap and stroke each other. This interaction may calm the female, suppressing her predatory instincts; it may communicate more about male fitness; or it may simply position the female correctly. If the female accepts the male’s advances, he’ll use special spurs on his forelegs to immobilize her fangs and bend her backwards. He then inserts his pedipalps into her genital opening, discharges his sperm, and beats a hasty retreat. There are other females to find and court.

Females may mate with more than five males per season. The sexual cannibalism seen among other spiders is rare, possibly because male and female Bay Area blond tarantulas are similar in size. Females would risk death by attempting to kill and eat their mates.

After mating season ends in mid-fall, the male soon dies. (By contrast, in captivity, females can live for more than 30 years.) The female stores sperm in organs called spermathecae and retreats to her burrow for the winter. Sometime the following spring or summer, she lays eggs, wraps them in silk, and begins periodically bringing the egg sac to warm in the sun at the edge of her burrow. After nearly two months, the eggs hatch and the spiderlings leave to dig their own burrows. Seven autumns later, the males among them set out to wander the hills like their fathers, their iridescent hair shining in the fading sunlight.

Where to See Tarantulas

Aphonepelma iodius is one of the most widely distributed tarantulas in California. It is found in the coastal mountain ranges south of the San Francisco Bay; in the hills west of the Central Valley; across the Transverse Ranges that run between Santa Barbara and San Bernardino; and in the Mojave Desert. The best places to see tarantulas include Mount Diablo, Sunol Regional Park, and Henry Coe State Park, which often host special events during mating season. The Mount Diablo Interpretive Association holds guided hikes. And Sunol Regional Park has tarantula-viewing treks. This year’s annual Tarantula Fest will happen at Henry Coe State Park on October 6.

About the Author

Ralph Washington, Jr. has a master’s degree in entomology from UC Davis, and he is a two-time national and international champion of entomological natural history trivia contests. He has been an enthusiastic student of arthropods since early childhood. His favorite thing about small creatures is that studying their lives often provides helpful lessons for his own. On the right evening in fall you might find him on Mount Diablo, watching eight-legged wanderers take a journey of great risk for a chance at life’s greatest reward.

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