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Published on Friday, September 22,
2023
By Elizabeth Anne Brown
A Costa Rican pirate spider lives up to the family name: It tricks closely related orb weaver spiders into “walking the plank,” right to their doom.
The world’s many pirate spiders exploit a very particular food source — other spiders. But while most pirate spiders invade the webs of other arachnids, Gelanor siquirres dupes potential prey into building a web right into a trap, researchers report in an upcoming Animal Behaviour.
Like any respectable pirate, pirate spiders have an extensive bag of tricks. Some species delicately strum the threads of other spiders’ webs to convince the arachnids they’ve caught an insect, only to strike when the web owner comes to collect its prey. Others mimic on a web the signature rhythms of a different spider’s courtship dance, luring would-be suitors to their deaths.
On a trip several years ago to a biological reserve in Costa Rica, researchers were the first to witness a hunt by a little-known species called G. siquirres. It cleverly exploits the way other spiders make their webs to get a meal, the team realized.
At nightfall in the steamy lowland rainforests of Costa Rica, orb weavers let loose “floating lines” — single strands of silk that blow in the breeze until the free end sticks on another surface, such as a tree branch. The orb weaver then scurries across to secure the second anchor point, and this first moored line serves as the foundation for the web.
But when G.
siquirres strikes, the orb weaver does
not scuttle across this bridge line to
some welcoming vegetation. Instead, it
walks into a trap; G. siquirres has
already cast its own silk lines, not
with the intention of building a web,
but to intercept that of the orb weaver.
The group also noticed another spider’s floating line caught on one of those strands. Then, as the floating line’s owner — a juvenile orb weaver — scuttled across, a pirate spider emerged from behind its leaf hideout and crept down toward the intersection.
Probably feeling vibrations from the approaching pirate spider, the orb weaver attached a dragline — the arachnid’s version of a bungee cord — and flung itself off its bridge line. The pirate spider was close on its heels, pursuing from its own dragline.
“They’re hanging there, fighting,” says Segura-Hernández, who recently finished her Ph.D. at the University of Nebraska–Lincoln. It was a dramatic scene, she says, like two swashbuckling pirates exchanging blows while swinging from the rigging. The encounter ended with a surprisingly swift attack from the pirate that instantly immobilized its prey.
The
hunting strategy comes as a total
surprise, says Gustavo Hormiga, a
biologist at George Washington
University in Washington, D.C., who
helped name and describe G. siquirres
in 2016. Exploiting how other spiders
build their webs is “actually pretty
sophisticated,” he says. “I don’t know
of anything like that in any [other]
spider group.” This new study helps bring G. siquirres to life, Hormiga says. Until now, the spider was known to science through only a dead specimen collected in 1994.
The
finding also underscores the need for
old-school natural historians, Hormiga
says, the kind of scientists who
wander out at night with a head lamp
to see what there is to find. “For
many of us, this is really what
brought us into science,” he says.
“Not pipetting. Just being out there.”
Elizabeth Anne Brown is a writer for Science News, which was founded in 1921 as an independent, nonprofit source of accurate information on the latest news of science, medicine and technology.
On the Wild Costa Rica page, you might discover more about the fascinating species that make the Pura Vida land one of the world's countries with the richest natural diversity.
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