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September 2018
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ONLINE NEWSLETTER
 
IT'S A TRAP!

By Jerry Roberts

Trapdoor spiders need to get out-of-doors more – Several groups of sedentary homebodies on Catalina Island need to get out from under their rocks to make connections.

Known to science as Bothriocyrtum californicum, the big, pudgy, subterranean trapdoor spiders could use some genetic variability for future sustainability of their groups – around Avalon, at Toyon Bay, the Airport in the Sky and Little Harbor.

Findings by Martina Ramirez, Ph.D., a professor of biology at Loyola Marymount University, suggest that while the trapdoor population at several California locations, including on Catalina and other Channel Islands, is generally varied, “bottlenecks do occur and populations are usually genetically isolated.”

“To foster preservation of the existing gene pool,” she told the audience at last year’s annual Catalina Island Conservancy symposium in Long Beach, “management of B. californicum should focus on maintaining as many populations…as possible and facilitating connections between them, while also creating or restoring habitat for potential colonization.”

At this point, Catalina’s trapdoor spiders aren’t in any danger. But the spiders aren’t helping their cause. Babies usually stay in the same area as the mother spider, dig their own homes nearby and mate locally. Staying close to home limits their genetic diversity and adaptability.

A typical trapdoor spider burrow is eight to 10 inches deep with enough room for the inhabitant to turn around and has a tight-fitting rock on top. A trapdoor spider detects the motion of close-crawling bugs and quickly tosses back the “door” to pounce on its meal. It drags dinner into the burrow and replaces the rock.

They feed on bugs seized outside their burrows and grow to the size of a computer mouse. A female spider can grow to be two inches in length and up to an ounce in weight. Trapdoor spiders can live up to 20 years – a phenomenal age for an invertebrate. But they will spend their lives mostly on one hillside.

Catalina’s historic situation has not been the best for the underground spiders. Ramirez says that intermittent fire suppression on the Island, as well as big mammals, such as non-native livestock prior to removal, may have collapsed many of the spiders’ burrows and cut off populations from one another.

Wasps are the trapdoor spiders’ only known enemy on Catalina. Some species of wasps tear the rocks off spider burrows, then sting and paralyze the inhabitants to insert larvae into the victims. The immobile yet living spiders then serve as hosts to feed the young wasps. In Central and South America, bird-sized wasps called colossal tarantula hawks, paralyze big spiders in the same manner.

“Wasps are out on the Island,” Ramirez said. “I’ve run into instances of parasite impact. The wasp cocoon is like a cigar sitting in the burrow.” Inside the cocoon is the drained husk of a dead spider.

As far as any danger to humans, the trapdoor spiders are minimally venomous. “I’ve been bitten,” Ramirez said. “You get a red bump, and then it goes away. Others are more sensitive to bites.”

“These guys,” she says, referring to the spiders, “have fangs, and they can easily puncture the skin. But they would never bother anyone unless you went out and dug them up out of the ground.”

Ramirez’s symposium presentation was entitled “Genetic Diversity among Island and Mainland Populations of the California Trapdoor Spider Bothriocyrtum californicum (Araneae, Ctenizidae).”

Jump Around, Jump Around

THE ISLAND NATURALIST
Issue #38 / All About Catalina's Spiders

IN THIS ISSUE...


Catalina’s Crabby Invertebrate
Jump Around, Jump Around
Fact or Fiction: Black/Yellow Argiope - nocturnal?
Did You Know…Summer naturalists weave web of knowledge




Frank Starkey

Trapdoor spiders fatten up on bugs, which they surprise by darting from under their manhole-like little rock covers.

Missed recent issues?
Issue #1 / All About Bison

Issue #2 / All About Birds

Issue #3 / All About Plants
Issue #4 / All About Eagles
Issue #5 / All About Ravens & Crows
Issue #6 / All About Natives & Invasives  
Issue #7 / All About Rain
Issue #8 / All About Bison Roundup
Issue #9 / All About Foxes
Issue #10 / All About Weeds
Issue #11 / All About Eagle Hatchlings
Issue #12 / All About Snakes
Issue #13 / All About Diurnal Raptors
Issue #14 / All About Diurnal Raptors II
Issue #15 / All About Giants & Dwarves
Issue #16 / All About Fire Ecology
Issue #17 / All About Mule Deer
Issue #18 / All About Feral Cats
Issue #19 / All About Acorn Woodpeckers
Issue #20 / All About Tachi the Fox
Issue #21 / All About Observing Nature
Issue #22 / All About Bald Eagle Update
Issue #23 / All About Invasive Plants
Issue #24 / All About Poisonous Plants
Issue #25 / All About the Value of Nature
Issue #26 / All About Edible Invasives
Issue #27 / All About Plants in Summer
Issue #28 / All About Marine Ecosystems
Issue #29 / All About Dominant Rocks
Issue #30 / All About Catalina’s Tongva
Issue #31 / All About Wildlife Fast Facts
Issue #32 / All About Wrigley Memorial
Issue #33 / All About Endemic Species
Issue #34 / All About Conservancy Volunteers
Issue #35 / All About Catalina’s Wildflowers

Issue #36 / All About Catalina’s Migratory Birds
Issue #37 / All About Reptiles & Amphibians

 

 

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