Pirate spiders ambush prey by tricking them with lines of silk

A species of pirate spider in Costa Rica has a hunting strategy that has never been documented before in any spider.

A species of pirate spider in Costa Rica has a unique hunting strategy: It attacks other spiders that use its silk lines as scaffolding for their own webs.

A pirate spider resting upside down on a leaf in Costa Rica
Roman Willi / naturepl.com

Many spiders build webs to snare prey, but their web design and purpose can vary widely depending on where they live and what they hunt. Some webs are flat sheets, while others form tube-like funnels or spiralling orbs. Other species forgo web-building altogether, and instead chase down or ambush prey.

Of the roughly 200 documented species of pirate spiders, most follow the same general hunting plan: They pluck strands of their prey’s web to mimic a trapped insect. When the prey spider comes to investigate, the pirate spider sneaks from its hiding place and strikes. Typically, “they walk subtly onto a spider web and attack the spider when it’s in reach”, says Laura Segura-Hernández at the University of Nebraska-Lincoln.

But the species Gelanor siquirres seems to take an even bolder strategy, as Segura-Hernández and her colleagues discovered while watching the spiders one evening in Costa Rica in 2016. The researchers found webs built by six different G. siquirres spiders, each with long, dry silk lines that extended down from plants – often anchored to the underside of a leaf – to the forest floor.

These roughly two-metre-long lines are tempting scaffolding for web-building spiders, which try to attach their floating lines to the dry silk each night. When the prey spider goes to attach their sticky line to the vertical one, the pirate spider senses the vibrations and drops down from their roost to attack.

“It was surprising to be able to record a whole new behaviour, and on a species that we know nothing about,” says Segura-Hernández. The new-to-science strategy could be particularly effective in tropical rainforests, which host lots of nocturnal web-building spiders that begin to build their webs at twilight, just after pirate spiders have deployed their vertical silk lines.

Next, the team plans to investigate whether this hunting strategy is unique to G. siquirres. Because scientists know relatively little about pirate spiders, it’s possible that other species have novel hunting methods waiting to be discovered.

Journal reference:

Animal BehaviourDOI: 10.1016/j.anbehav.2023.07.001

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