Defensive spray by a semiaquatic osmylid larva (Insecta: Neuroptera) for both aquatic and terrestrial predators

Abstract

Chemical secretions are an effective means by which insects can deter potential enemies. Several terrestrial insects spray these liquids directionally toward enemies, but little is known about spraying behavior in aquatic and semiaquatic insects. The larvae of Osmylus hyalinatus (Neuroptera: Osmylidae) are semiaquatic, inhabiting the edges of small streams and ponds where they encounter multiple enemies on land and in water. The larvae of this osmylid sprayed a hyaline liquid from the anal opening if disturbed in either air and water, although the spray appeared slightly viscous in water. The liquid was stored in the posterior half of the hindgut and sprayed directionally toward an artificial stimulus. Spraying allowed the larvae to escape biting by ants, and to repel them in 90% of encounters. Spraying caused the regurgitation of 71% and 60% of all larvae swallowed by terrestrial frogs and aquatic newts, respectively. Aquatic fishfly larvae released 30% of captured larvae due to spraying. Most of the larvae that repelled ants or were regurgitated by amphibians survived, but those released by fishfly larvae were killed by heavy biting with the mandibles. This is the first report of effective liquid spraying by insects in water, and also within the order Neuroptera.