The bimodal gas exchange strategies of dragonfly nymphs across development

Dragonfly nymphs are aquatic and breathe water using a rectal gill. However, it has long been known that the nymphs of many species appear to possess the ability to breathe air, either during their final instar when they leave the water prior to metamorphosis, or during periods of aquatic hypoxia. The aerial gas exchange associated with these activities has not been quantified. This study used flow-through respirometry to measure the rate of aerial CO₂ release (V̇CO₂) from dragonfly nymphs as a proxy for their aerial gas exchange, both across development and in response to progressive aquatic hypoxia. It examined a total of four species from two families (Libellulidae and Aeshnidae). In both families, the late-final instar nymphs developed functional mesothoracic spiracles, allowing them to breathe air by positioning their head and thorax above the water's surface. While breathing air in this position, the nymphs could also ventilate their submerged rectal gill. Thus, during bimodal gas exchange in normoxic water, it was calculated that aeshnid nymphs expelled 39% of their respiratory CO₂ into the air through their spiracles, while libellulid nymphs expelled 56% into the air. Decreasing the aquatic PO₂ to 2.5 kPa and then below 1 kPa increased the proportion of respiratory CO₂ expelled into the air from 69% to 100%, respectively. Thus, bimodally breathing late-final nymphs can vary how they partition gas exchange between their spiracles and their gill depending on aquatic PO₂. Aeshnid nymphs of all developmental stages were also found to use their rectal gill as an air-breathing organ; pre-final nymphs performing 'surface skimming' while late final nymphs aspirated air bubbles directly into their gill's branchial basket. Mass-specific rates of aerial V̇CO₂ also increased as the nymphs approached metamorphosis. These findings indicate that aeshnid nymphs are capable of accessing aerial O₂ across development using their rectal gill as an air breathing organ, while the aquatic nymphs of both aeshnid and libellulid dragonflies undergo a progressive shift towards using the atmosphere for respiration as they approach metamorphosis.