Novel Strategies in the Complex Defense Repertoire of a Stonefly (Pteronarcys dorsata) nymph

Abstract

The nymph of Pteronarcys dorsata Say, a large and primitive stonefly, exhibits a range of behavioural defenses. We studied predator-prey interactions between stonefly nymphs and co-existing pelagic (trout) and benthic predators (sculpins, suckers and crayfish) to determine whether the defenses used by P. dorsata were predator specific. Nymphs of P. dorsata distinguished between different predator types and applied an appropriate defense mechanism to effectively avoid predation. To benthic and pelagic fish predators, P. dorsata responded primarily by freezing and thanatosis (feigning death). Survival was lower in the stonefly species Agnetina capitata which did not display thanatosis after attacks by trout. Nymphs of P. dorsata responded to benthic tactile crayfish predators by retreating and reflex bleeding (autohemorrhaging) which allowed for escape in all observed encounters. Reflex bleeding has never before been demonstrated as a defense mechanism for insects in aquatic environments. Risk of predation increased in nymphs deprived of their behavioural defenses. Stonefly nymphs were aggressive to conspecifics and experiments suggested that this behaviour may be a mechanism to avoid predation through spacing. Complex anti-predator defenses are generally unknown in aquatic insects; their discovery in such a primitive insect indicates that such behaviours may be evolutionarily ancient.