Choosing substrates with or without predator cues by <em>Prionostemma</em> Opiliones

We experimentally tested for the avoidance of predator cues in an arachnid that relies on contact chemoreception and frequently loses its legs, the Neotropical Opiliones in the genus Prionostemma. First, we tested whether these arachnids can detect and avoid chemical cues derived from potential predators. When choosing between leaf litter substrates with or without the urine of a sympatric predator (the toad Rhinella horribilis), individuals preferred the substrate lacking the predator cue. Second, we tested that substrate choice would be affected by previous leg loss, given a potential reduction in sensory detection with fewer sensory organs. However, we found that individuals with autotomized legs were just as likely to avoid substrates with predator chemical cues as intact individuals. Methods We collected Opiliones of Prionostemma sp.1 from their daytime roosting aggregations in La Selva Research Station in Sarapiquí, Costa Rica. We brought them to the lab and house in terraria. We then prepared the chemical cue of a potential predator (hereafter referred to as the “predator cue”), we obtained urine from one Rhinella horribilis toad the night before collecting Opiliones. We spread the toad's urine in leaf litter and place the urine-infused plant material in half of a testing arena. The other half of the arena was control leaf litter that was infused only in water. We conducted trials in which we presented both stimuli (leaf litter with and without predator cues) to Opiliones. In each trial (n = 31), we placed five individuals in the testing arena with the two-stimuli setup. We placed the five individuals one by one in the center of one of the arenas, ensuring and confirming that each individual touched both the control and predator substrates at the moment of release. This way, we ensured that each individual was in contact with (and potentially aware of) both cue options from the start of the trial. We then covered the arena with mesh to prevent escape and permit observation. After 5 minutes (enough time for all individuals to settle), we recorded the number of individuals on each substrate type, as well as the number of legs of each individual.