Name: Antrodiaetus riversi. Turret spiders are named for the small tower-like structures they build.
Classification, Range and Habitat:These spiders are found only in California, in the Coast Range and Sierra foothills. Within their range, they are limited to moist woodlands, often on north-facing slopes, and near shady streams and thickets.
Their susceptibility to dehydration limits their ability to disperse, so populations become isolated, and diverge over time. Although distant populations may look the same to human eyes, modern molecular biological identification methods indicate that there are at least eight species of turret spiders. They belong to an ancient lineage of spiders, the mygalomorphs, which includes some of the world’s largest and longest-lived spiders. Tarantulas and trapdoor spiders are also part of this group, sometimes called “primitive spiders” (as opposed to true or modern spiders). Perhaps the most obvious difference is mygalomorph fangs swing straight down, instead of towards each other from the sides like pincers. In general mygalomorphs have heavy builds, and don’t hang in silk webs; they spend the bulk of their time in burrows.
What to Look for and Notice: Full-grown, turret spiders are only about ¾-inch long, and they generally hide in the ground in daytime. But you can look for their tiny turrets! They’re usually camouflaged by bits of plant debris, so the first thing your eyes might pick out are holes in the ground (up to half-inch or so in diameter). Keep looking to notice a collar or tube, made of soil and plant bits, that extends roughly perpendicularly above the soil surface, sometimes by an inch or two. Other creatures, such as tiger beetle larvae, also live in burrows, but their entrances are flush with the soil surface and unadorned. The spider’s silk-lined burrow can be up to 8-in. deep. The silk lining is thicker nearer the top, and gradually thins to the bottom. This thicker silk provides support where soil is less compacted, and also helps incorporate debris for building the turret. The spiders build with whatever materials are handy. Where available, tiny twigs and pine needles might radiate outward, spoke-like, from the turret, which increases the spider’s ability to sense prey. Turrets also help keep rain out of the burrow. But turrets have also proved flexible and fragile to scientists trying to measure them, so they are best left alone.
What They Eat and Who Eats Them: Like many of their mygalomorph relatives, turret spiders are ambush predators that lurk in burrows. Turret spiders wait deep in their burrows during the day, and come up for food at night. poised just inside the turret. They capture millipedes, ants, termites, beetles and other arthropods. They sense prey through vibrations, quickly lunging part-way out of the turret to seize whatever stumbles past. The last pair of legs grips the turret lip, and the first two pairs of legs help the mouthparts grab and position the prey for piercing with the fangs. Once injected with venom, the meal is pulled down into the burrow. If prey gets away, turret spiders don’t give chase. One researcher who wanted to study turret spiders more closely in the lab tried to entice them from their burrows with tethered prey. It didn’t work, and he had to dig out the spiders.
Any creature that would eat a spider that size is a potential turret spider predator. But the protective burrow makes it a harder meal to find. As is often the case, the dispersing young are most vulnerable. The burrow provides little protection from a parasitoid fly that uses turret spiders as a host.
Roles in Ecosystem: These tiny predators help control populations of other arthropods, and no doubt end up feeding some of them too. Although they’re seldom seen, the homes they build delight nature-loving humans.
Life and Behavior: Turret spiders are homebodies, who rarely leave their burrows, and for most of the year each burrow houses just one of the dark brown spiders—except when young spiderlings remain in their mother’s burrow during their first winter. Mating occurs in August or September, and it’s the only time adults —males only— leave their burrows. When a male reaches sexual maturity (at 8 or 9 years!), he sets out at night to find a female and mate, or die trying. He dies afterwards, anyway. Females continue to mate yearly, and can reach at least 16 years of age.
Silk-wrapped egg sacs, attached half way down the burrow, contain 20-70 eggs. Spiderlings hatch in early fall, and usually wait to disperse until spring. They can’t go very far, because they’re small and dehydrate easily, so they dig in nearby, building their burrows close to their mother’s. That’s why spider turrets occur in clusters of smaller burrows around a larger burrow of an adult female. It’s a family thing.
Turret Spiders at Edgewood
- Where at Edgewood am I likely to see turrets? You are most likely to find spiders’ turrets along the Sylvan and Baywood Glen Trails in shady oak woodlands. Search the banks cut into the hill on the upper side of a trail for the distinctive, dark opening of the turret. Once you’ve spotted a larger turret, look nearby for the smaller turrets of younger spiders. Don’t touch or disturb any turret (or the homes of any other animals)!
- Will I be able to see the spiders? Turret spiders stay out of sight in their turrets in the day time. Careful observers may spot the spiders near the tops of their turrets around dusk. Edgewood Park and Natural Preserve is closed at night, but Friends of Edgewood occasionally offers docent-led night walks. Check our Events page.
James Starrett and Marshall Hedin (2007) Multilocus genealogies reveal multiple cryptic species and biogeographical complexity in the California turret spider Antrodiaetus riversi (Mygalomorphae, Antrodiaetidae) Molecular Ecology (2007) 16, 583– 604
Leonard S. Vincent (1993) The Natural History Of The California Turret Spider Atypoides Riversi (Araneae, Antrodiaetidae): Demographics, Growth Rates, Survivorship, And Longevity The Journal of Arachnology 21:29—39
Based on an article by Carolyn J. Strange. Science writer Carolyn J. Strange has written hundreds of articles. She became an Edgewood neighbor in 1998, a Friends of Edgewood docent in 2003, and has served the Friends of Edgewood in various ways ever since.
All photos taken at Edgewood Park and Natural Preserve. Used with permission.