Eco-mechanics of lamellar autotomy in larval damselflies

In larval damselflies, the self-amputation (autotomy) of the caudal lamellae permits escape from predatory larval dragonflies. Lamellar joint size declines among populations with increasing risk of dragonfly predation, but the breaking force required for autotomy and the biomechanical factors that influence breaking force are unknown. If autotomy enhances survival in larval damselflies, then predation by larval dragonflies should select for joints that require less force to break. We test this adaptive hypothesis by evaluating whether breaking force is negatively related to local predation risk from larval dragonflies. We also test a cuticle structure hypothesis, which predicts that breaking force is positively related to joint size and to joint cuticle thickness because of a structural support relationship between joint and lamella. The peak force necessary for lamellar autotomy was assessed on individual larval Enallagma damselflies collected from populations that varied in risk of predation. Easier lamellar autotomy occurred in larvae from sites with higher predation risk because damselflies from fishless ponds (where predatory larval dragonflies are likely more abundant) had lower breaking forces than those from ponds with fish (where larval dragonfly predation is likely reduced). Furthermore, breaking force was a positive function of joint size and also of total cuticle cross-sectional area after controlling for joint size. This suggests that autotomy may evolve in larval damselflies under selection from small grasping predators such as larval dragonflies by favouring smaller joint size or reduced cuticle area of lamellar joints.