Investigating Respiration and Gas Exchange in the Aquatic Bug Aphelocheirus aestivalis (Hemiptera: Aphelocheiridae), and Surface Dwelling and Subterranean Diving Beetles (Coleoptera: Dytiscidae)

Abstract

Insects evolved in a terrestrial environment as indicated by their gas filled respiratory system, the tracheal system. Those insects that have become aquatic secondarily contend with the challenge of interfacing their tracheal system with the water, where O2 diffuses 250,000 times slower than through air. These aquatic insects use a range of mechanisms to overcome this challenge, including gas exchange through the body surface (cutaneous respiration), siphons/snorkels that provide a direct link with the atmosphere, and air bubbles on the surfaces of the insects that store O2 (air stores) or passively gain O2 from the water (gas gills). Studies within this thesis investigate respiration, gas exchange, and dive behaviour in the aquatic bug Aphelocheirus aestivalis (Hemiptera) which uses an incompressible gas gill (plastron), and diving beetles (Dytiscidae: Coleoptera) that use air stores, gas gills, and varying forms of cutaneous respiration while underwater. Chapter two investigates under what temperature and convective conditions the metabolic demands of Aphelocheirus may become O2-limited, and it compares these to the ecological niche in which the species can survive. Chapter three tests the hypothesis that subterranean diving beetles use cutaneous respiration to help them to survive in subterranean environments where few or no air-water interfaces occur. It also tests if cutaneous respiration in these beetles limits them to small size. Chapter four investigates respiration, gas exchange and dive behaviour in a surface-dwelling diving beetle Platynectes decempunctatus. This study develops an understanding of how gas exchange relates to dive behaviour and quantifies aspects of the dive including the decline in O2 pressure within the respiratory gas store and the amount of O2 gained from the water. Chapter five investigates respiration, gas exchange and dive behaviour in diving beetles more broadly, undertaking allometric analyses with several surface-dwelling diving beetles across a >300-fold mass range. This study shows the relationships between beetle size, O2-consumption rate, respiratory gas volume held during a dive and dive behaviour. The data from this and previous studies are then incorporated into a mathematical model to assess four modes of gas exchange with the water in dytiscids, including cutaneous respiration, setal tracheal gills, respiratory pores and gas gills. There is a common theme throughout the research chapters of undertaking a range of experiments and then developing a mathematical model based on Fick’s general diffusion equation. This approach enables a deeper understanding of the underlying mechanisms and allows model manipulations to determine the limits of the respiratory modes and their ecological and evolutionary consequences.