Recent observations out of Madagascar suggest that wily spiders may be luring frogs into traps constructed from tree leaves – if true, it's an impressive new example of the boneless taking out the backboned.

A huntsman spider dining on a tree frog inside its leafy retreat. Image © Dominic Martin

The findings were announced in a paper in Ecology & Evolution released last month. The study team – composed of researchers with the Regional University Centre of the SAVA Region and the University of Antananarivo in Madagascar and the University of Goettingen in Germany – conducted fieldwork amid deforested, agriculture-dominated landscapes of northeastern Madagascar, around Ambodiala and Antsikory.

In an overgrown fallow field, the researchers found a huntsman spider of the genus Damastes snacking on a tree frog: Heterixalus andrakata, to be exact, one of the endemic Madagascar reed frogs. Finding itself scrutinised, the huntsman dragged the frog’s corpse into a close-by hideaway formed of leaves partly sealed together with spider’s silk: a sort of pouch.

Following this initial observation, the team discovered other, similar leaf pouches scattered in vanilla plantations in the region. Though they were fashioned in a number of different tree species, the structures were all similar, with a pair of live, green leaves fused with silk along their edges and sometimes also at their tips (or apices), leaving an opening at the leaf bases. Damastes spiders were found sequestered inside, “well-hidden at the rear end of the trap (i.e. the apex of the leaves),” the study authors wrote, “and not visible from the entrance.”

Damastes sp. concealed in a leaf pouch. Image © Fulgence Thio Rosin

A broad survey published last year in Global Ecology & Biogeography looked at predation by arthropods – arachnids, insects, and crustaceans as well as centipedes and other myriapods – on backboned critters and declared them “underestimated predators of vertebrates.” The review found spiders the foremost vertebrate slayers among the arthropods, the leading predator of all vertebrate groups except birds (which, apparently, have most to fear from praying mantises – among a hummingbird’s worst nightmares). And some 40 percent of the vertebrates falling victim to arthropods, according to the review, were amphibians – “specifically frogs.”

The team behind the Madagascar study notes in the Ecology & Evolution article that Heterixalus andrakata seeks out shady hideaways in the heat of the day, and suggest that the leaf pouches built by the huntsman spider may be designed to appeal to the tree frogs for that purpose – and perhaps, all the more perversely, as a refuge from predators. While many huntsman spiders, as their name suggests, rove about actively in search of prey, those of the Damastes genus commonly use a sit-and-wait ambush strategy to nab victims, and the leaf pouches—a variation of the silk-secured debris nests some huntsman spiders shelter in – may represent a specially refined frog death-trap.

A huntsman spider dining on a Heterixalus andrakata tree frog. Image © Fulgence Thio Rosin

The authors stress more research needs to be done, especially given they observed only one, already-dead frog being eaten by a spider. They suggest, however, that, if their hypothesis is correct, their findings may reveal a heretofore-unappreciated level of frog-catching strategy by spiders. “Previous reports of spiders preying on amphibians point to an opportunistic behavior and provide no evidence of specialization,” they write. “Based on our report, we speculate that the spiders used targeted traps to prey on amphibians.”

As we – compassionate fellow vertebrates that we are – ponder this potential horror that Madagascar tree frogs have to deal with, let’s thank our lucky stars (as perhaps we should on a daily basis) that spiders aren’t any bigger than they are.