Scientists Think Cave Spiders Build Tripwire-Like Mechanisms To Ensnare Prey

The European cave orb spider (Meta menardi) inhabits the caves of western and central Europe, though they can be found lurking in other dark, damp habitats such as sewers, bunkers, railway tunnels, and hollow trees. The dingy nature of these habitats and the impact they have on the spiders’ diets may be behind the unusual tactics they employ to catch their prey.

Orb spiders typically spin circular webs – hence the name. These constructions ensnare flying insects like mosquitoes, flies, and winged ants. However, flying insects such as these are less integral to the diet of M. menardi, making up only a half or so of what they eat. The rest comprises centipedes, slugs, and crawling insects, as well as other spiders.

As such, this little arachnid has had to adapt its hunting methods. Unlike its fellow orb spiders, the web of the M. menardi tends to be smaller and contains fewer frame strands. Instead, it has more radial threads that serve as anchors and attach the structure to the cave wall. Whereas the webs of spiders found above ground tend to have five to 10 attachment points, those of the M. menardi have between 10 and 20. 

Up until this point, researchers have been unsure as to how exactly the web works to trap crawling prey. There have been several theories posited, including the off-web hunting hypothesis (the spider leaves the web to catch prey) and the prey slip hypothesis (the prey slips off the cave wall and falls into the web). Now, new observations recorded by researchers from the University of Oxford put weight behind the radius tripwire hypothesis (the web produces vibrations that alert the spider to the presence of prey).   

Observations took place in Mother Grundy’s Parlour, a quirkily named cave in the Creswell Crags Archaeological Park in Derbyshire, UK. Researchers watched as prey – a mealworm, centipede, and M. menardi – touched the web of a European cave orb spider. They then observed the spider’s behavior. 

In each of the three observations, the spider responded by running along a neighboring radius, rather than the radius that had been directly touched by the potential prey. According to the researchers, this may be a smart tactic that allows the spider to approach the prey from behind, though they do acknowledge that it could be a genuine mistake. 

Equally, while the observations themselves provide evidence in favor of the tripwire hypothesis, the study’s authors caveat that the attempts by the spider to claim the prey were unsuccessful in each case. This may be because the prey themselves were a potential threat – indeed, a centipede and a fellow M. menardi could pose a danger to the resident spider. In short, they say, more observations are required to confirm this hypothesis and it may well be that there is truth to each of the hypotheses put forward.

Nevertheless, this is not the only unusual tactic deployed by an orb spider. The araneid bolas spider releases pheromones that mimic those of a female moth to lure in males, while the Araneus ventricosus manipulates the flashes of fireflies to attract more into their web.

The study is published in the journal Ethology.