The European cave spider (Meta menardi) builds orb webs in the entrance and twilight zone of caves. However, the scarcity of flying prey inside caves means that only about half of their diet consists of flying insects with walking prey making up the remaining half. The capture of non-flying prey is not generally known from aboveground orb web building spiders, who exclusively captures flying prey in their web. It is currently a mystery how the cave spider manages to capture prey walking on the cave wall. A number of hypotheses have been suggested including that the spider leaves its web completely (the off-web hunting hypothesis), that walking prey slips on the smooth cave ceiling and falls into the horizontally-inclined webs (the prey slip hypothesis), or that the unique geometry of the cave spider’s orb web, which lacks frame threads, means that the radii that attach directly to the cave wall are used as tripwires to alert the spider of passing prey (the radius tripwire hypothesis). Here we report on three in-situ observations of M. menardi responding to radius stimulation from artificial and walking prey by running towards the cave-wall. Although the opportunistic observations of responses to real prey both involved potentially dangerous prey (a centipede and another large M. menardi) and were unsuccessful prey capture events, the clear response to walking prey nonetheless constitutes the first strong observational evidence in support of the radius tripwire hypothesis.
