Cost of defensive spraying by larval Osmylus hyalinatus (Neuroptera: Osmylidae) for post-larval development

The chemical defense of insects is effective for avoiding predation, but may carry a cost in terms of life history traits. If chemical defenses require the resources and/or nutrients necessary for the larva or post-larval stages to survive, grow, and reproduce, there will be a trade-off between chemical defense and other traits, particularly in habitats where larvae are subjected to frequent predator attacks. The larvae of Osmylus hyalinatus McLachlan (Neuroptera: Osmylidae) are semiaquatic, inhabiting the edges of small streams and ponds, where they encounter multiple predators both on land and in water. Larvae of this species spray a hyaline liquid from an anal opening when disturbed. The liquid is stored in the posterior half of the hindgut. Daily stimulation of larvae to exhaust the stored liquid, thereby simulating repeated predator attacks, resulted in smaller adult body size at emergence than the control, but had little effect on the larval/pupal period, cocoon production (for predator avoidance of prepupae and pupae), reproductive potential, or chemical defense of adults in which prothoracic glands release a substance that smells unpleasant to predators. The lack of such effects is explained in part by adults gaining more resources through feeding than the larval stages, as well as nuptial gifts from males to females. The spraying liquids and silk used to spin cocoons are both discharged from an anal opening; therefore, a trade-off between these two materials is plausible and should be examined in the future.