Lacking sensory (rather than locomotive) legs affects locomotion but not food detection in the harvestman Holmbergiana weyenberghi

The ability to release a leg when forced by predators or during agonistic interactions is widespread and frequent in arthropods. Despite immediate benefits, losing legs may affect locomotion, sensory performance, reproduction, and fitness. The costs of autospasy in arachnids have been scarcely addressed. Therefore, we tested the hypothesis that the number and type of self-amputated legs (sensory or locomotive) affect locomotion and food detection speeds in the harvestman Holmbergiana weyenberghi (Holmberg, 1876) (Sclerosomatidae). With field surveys in a subtropical forest in Uruguay we found that 35% of individuals lacked at least one leg, and sensory legs (second pair) were the most frequently lost. In an indoor setup, we found that individuals missing one sensory leg walked and climbed a trunk slower than individuals lacking a locomotive leg (first, third, or fourth pair), or compared with those with eight legs. Lacking legs did not affect the food detection speed. Additionally, larger individuals with eight legs had greater walking and climbing speeds. Therefore, losing sensory legs affects locomotion in these harvestmen and may confer costs in orientation, balance, and substrate recognition. Finally, we compared our results with the different patterns reported for the effect of autospacy in other harvestman species.