Quantifying the acid-base status of dragonflies across their transition from breathing water to breathing air

Abstract

Amphibiotic dragonflies show a significant increase in hemolymph total CO2 (TCO2) as they transition from water-breathing to air-breathing. This study examines the hemolymph acid-base status of dragonflies from two families (Aeshnidae and Libellulidae) as they transition from water to air. CO2 solubility (αCO2) and the apparent carbonic acid dissociation constant (pKapp) were determined in vitro, and pH/bicarbonate [HCO3−] plots were produced by equilibrating hemolymph samples with PCO2 between 0.5-5 kPa in custom-built rotating microtonometers. Hemolymph αCO2 varied little between families and across development (mean 0.355±0.005 mmol l−1 kPa−1) while the pKapp was between 6.23 to 6.27, similar to values determined for grasshopper hemolymph. However, the non-HCO3− buffer capacity for dragonfly hemolymph was uniformly low relative to other insects (3.6 to 5.4 mmol l−1 pH−1). While aeshnid dragonflies maintained this level as bimodally-breathing late-final instars and air-breathing adults, the buffer capacity of bimodally-breathing late-final instar Libellula nymphs increased substantially to 9.9 mmol l−1 pH−1. Using the pH/[HCO3−] plots and in vivo measurements of TCO2 and PCO2 from early-final instar nymphs, it was calculated that the in vivo hemolymph pH was 7.8 for an aeshnid nymph and 7.9 for a libellulid nymph, respectively. The pH/[HCO3−] plots show that the changes in acid-base status experienced by dragonflies across their development are more moderate than those seen in vertebrate amphibians. Whether these differences are due to dragonflies being secondarily aquatic, or arise from intrinsic differences between insect and vertebrate gas exchange and acid-base regulatory mechanisms, remains an open question.