Just a few days ago we reported on how spiders are masters of not getting tangled in their own webs – but that doesn’t mean the silken traps are safe spaces for them. In the first known case of its kind, scientists have witnessed a spider being attacked and killed on its own web by a seemingly unlikely predator: a worm.
The spider, a comb-footed spider (Helvibis longicauda), was minding its own business and chilling on its web while guarding its egg sac, when the slow-moving, poorly sighted flatworm pounced. The scene was stumbled upon by João Cardoso, one of the authors on a recent study describing the unusual predation event, back in 2014 in Brazil’s Atlantic Forest.
After taking the two adversaries back to the lab for further study, Cardoso and co-authors realized the worm was a “land planarian” – a family of land-dwelling flatworms that prey on other invertebrates – belonging to the genus Choeradoplana. The spider, meanwhile, was found constricted and covered in sticky mucus, still clutching its egg sac.
A worm might not sound like the most fearsome of predators, but land planarians such as Choeradoplana are much more threatening than they seem. While some have a taste for soft-bodied animals, such as earthworms, others are less picky and can prey on many organisms, including arthropods – the group to which spiders belong.
To best their prey of choice, predatory flatworms rely on physical force and corrosive mucus, first wrapping around their prey to immobilize them, before dousing them in digestive fluids and snacking on the remains.
While the villainous turn from Choeradoplana may not be all that surprising, it’s still incredibly rare to witness flatworms interacting with their prey, and as such the authors believe this is the first case of predation of a web-building spider by a land planarian ever to be described.
“Our observation broadens the scope of possible natural enemies of web-building spiders and the prey items of land planarians,” the authors write in the study. “It also indicates that these organisms can capture and overpower dangerous predatory arthropods, suggesting that even complex three-dimensional sticky webs can be ineffective against the attack of land planarians.”
This, they go on to suggest, is likely because the fiendish flatworms’ mucus counters the web’s adhesive properties, allowing them to traverse the strands and access their prey.
We may be more accustomed to (though no less surprised by) spiders overpowering unlikely targets, but not so much the other way around.
“A worm, dominating a spider! It’s really amazing,” Cardoso told New Scientist.
The study is published in Neotropical Biology and Conservation.