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These spiders can sew leaves in fake shelters to lure frogs to their misfortune

On a sweltering afternoon in northeastern Madagascar, the freshness of the shade of a leaf is an attractive respite for a frog. But some of these oases may hide hungry architects: hunting spiders.

New observations show that gangly spiders partially bind two leaves together using silk, creating a leafy hollow. One of the arachnids was seen eating a frog inside one of its pockets, suggesting that spiders create the structures to attract and trap frogs, researchers reported on Dec. 11 in Ecology and Evolution.

In 2017 and 2018, biologist Thio Rosin Fulgence and colleagues were conducting an ecological research in Madagascar when Dominic Martin, an ecologist at the University of Göttingen in Germany, saw a large hunting spider (Damastes sp.) Eating a small cane frog from Madagascar (Heterixalus andrakata). The spider was on a small tree, about a couple of overlapping leaves that had been tied together with spider silk to create a pocket. As he approached, the spider retreated into its leafy lair, the prize of the amphibians in tow.

A hunting spider feeds on a Madagascar cane frog inside a pocket built of leaves and spider silk.Dominic Andreas Martin

“The first time we encountered this phenomenon we were very excited,” says Fulgence, of the University of Antananarivo in Madagascar. The following year, while conducting surveys of reptiles and amphibians in roughly the same region, Fulgence found three more of the spiders hidden in similar-leaf retreats. These spiders were not seen with prey, he says.

Some spiders are predators of larger, stronger vertebrates such as the mouse opossum and even frogs, if given the chance. (SN: 28/02/19). When this happens, arachnids are often seen as winners of the vertebrate grand prize. But hunting spiders, by contrast, may be specifically targeted at frogs as prey, researchers say. By joining the leaves, spiders are creating cool, dark microhabitats that would be desirable in a dry, scorching environment with many predatory birds, Fulgence says.

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Still, these spiders could simply hide in the leafy retreats and ambush prey passing by, not use the structures as traps, says Stano Pekár, a behavioral biologist at Masaryk University in Brno, Czech Republic, who did not participate in the research.

That is true, agrees Jose Valdez, a conservation biologist at the German Center for Integrative Biodiversity Research in Leipzig. But “what makes me think otherwise is that the researchers not only found (the leaf recedes) several times, but the spider wove the edges of the leaves,” says Valdez, who did not participate in the study. "I think there would be much easier places for these spiders to hide in a forest."

Part of the reason for the uncertainty is that the spotted spider eating a frog was first seen out of the pocket of the leaves when the arachnid was consuming its prey. “Only detailed observations and experiments” can confirm whether the leaves are a frog trap, says Rodrigo Willemart, a zoologist at the University of São Paulo, who also did not participate in the research.

If so, such a tool could be unique among spiders, Willemart says. "I don't know of papers that reported on traps built by spiders specifically to capture vertebrates."

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