Submerged oviposition and responses to oxygen lack in Enallagma cyathigerum (Charpentier) (Zygoptera: Coenagrionidae)

Female Enallagma cyathigerum (Charpentier) regularly oviposit below the water surface on submerged vegetation. While submerged they probably depend for oxygen on an air layer trapped on the body surface which is thought to act as a physical gill. Although they may attempt to maximise egg-laying during each period of submergence because of high predation and low survival rates, oxygen shortage is one factor which may cause them to surface while still containing some ripe eggs. Females float up and most become trapped in the meniscus where they depend on males for their rescue. Rescuing males are rewarded if females contain some ripe eggs, accept copulation and return to oviposit soon afterwards. The examination of females caught at the surface immediately after oviposition showed that 51% contained at least 10 ripe eggs in the lateral oviducts and 28% contained > 50 eggs. Females may attempt to prolong their submergence by making bodyrolling movements which enhance the oxygen supply through stirring the water in contact with their superficial air layer. These movements bear some resemblance to the shaking behaviour of hypoxic Calopteryx larvae, and the possibility of their common origin is discussed.