Changes in hemolymph total CO2 content during the water-to-air respiratory transition of amphibiotic dragonflies

Abstract

Dragonflies (Odonata, Anisoptera) are amphibiotic; the nymph is aquatic and breathes water using a rectal gill before transitioning to the winged adult that breathes air through spiracles. While the evolutionary and developmental transition from water- to air-breathing is known to be associated with a dramatic rise in internal CO2 levels, the changes in blood-gas composition experienced by amphibiotic insects, which represent an ancestral air-to-water transition, are unknown. This study measured total CO2 (TCO2) in hemolymph collected from aquatic nymphs and air-breathing adults of Anax junius, Aeshna multicolor (Aeshnidae), Libellula quadrimaculata, and L. forensis (Libellulidae). Hemolymph PCO2 was also measured in vivo in both Aeshnid nymphs and marbled crayfish (Procambarus fallax. f. virginalis) using a novel fiber-optic CO2 sensor. The hemolymph TCO2 of the pre- and early-final instar nymphs was found to be significantly lower than that of the air-breathing adults. However, the TCO2 of the late-final instar Aeshnid nymphs was not significantly different from the air-breathing adult, despite the late-final nymph still breathing water. TCO2 and PCO2 were also significantly higher in the hemolymph of early-final Aeshnid nymphs compared to the water-breathing crayfish. Thus, while dragonfly nymphs show an increase in internal CO2 as they transition from water to air, from an evolutionary standpoint, the nymph's ability to breathe water is associated with a comparatively minor decrease in hemolymph TCO2 relative to the air-breathing adult.