Gas exchange and dive behaviour in the diving beetle Platynectes decempunctatus (Coleoptera: Dytiscidae)

Many aquatic insects use bubbles on the body surface to store and supply O₂ for their dives. There are two types of bubbles: air stores, which store O₂ gained from air at the surface, and gas gills that allow passive extraction of O₂ from water. Many insects using air stores and gas gills return to the surface to replenish their bubbles and, therefore, their requirement for O₂ influences dive behaviour. In this study, we investigate gas exchange and dive behaviour in the diving beetle Platynectes decempunctatus that uses a sub-elytral air store and a small compressible gas gill. We measure the PO₂ within the air store during tethered dives, as well as the amount of O₂ exchanged during surfacing events. Buoyancy experiments monitor the volume of gas in the gas gill and how it changes during dives. We also directly link O₂-consumption rate at three temperatures (10, 15 and 20 °C) with dive duration, surfacing frequency and movement activity. These data are incorporated in a gas exchange model, which shows that the small gas gill of P. decempunctatus contributes less than 10% of the total O₂ used during the dive, while up to 10% is supplied by cutaneous uptake.

Cutaneous respiration by diving beetles from underground aquifers of Western Australia (Coleoptera: Dytiscidae)

Insects have a gas-filled respiratory system, which provides a challenge for those that have become aquatic secondarily. Diving beetles (Dytiscidae) use bubbles on the surface of their bodies to supply O2 for their dives and passively gain O2 from the water. However, these bubbles usually require replenishment at the water's surface. A highly diverse assemblage of subterranean dytiscids has evolved in isolated calcrete aquifers of Western Australia with limited/no access to an air–water interface, raising the question of how they are able to respire. We explored the hypothesis that they use cutaneous respiration by studying the mode of respiration in three subterranean dytiscid species from two isolated aquifers. The three beetle species consume O2 directly from the water, but they lack structures on their bodies that could have respiratory function. They also have a lower metabolic rate than other insects. O2 boundary layers surrounding the beetles are present, indicating that O2 diffuses into the surface of their bodies via cutaneous respiration. Cuticle thickness measurements and other experimental results were incorporated into a mathematical model to understand whether cutaneous respiration limits beetle size. The model indicates that the cuticle contributes considerably to resistance in the O2 cascade. As the beetles become larger, their metabolic scope narrows, potentially limiting their ability to allocate energy to mating, foraging and development at sizes above approximately 5 mg. However, the ability of these beetles to utilise cutaneous respiration has enabled the evolution of the largest assemblage of subterranean dytiscids in the world.

The effects of temperature, activity and convection on the plastron PO2 of the aquatic bug Aphelocheirus aestivalis (Hemiptera; Aphelocheiridae)

The aquatic bug Aphelocheirus aestivalis (Fabricius 1794) utilises a plastron, a thin bubble layer on the surface of its body to extract O₂ from the water. Millions of tiny hairs keep the bubble from collapsing, enabling the bug to remain submerged indefinitely. The development of fibre optic O₂-probes has allowed measurements of O₂ pressure (PO₂) surrounding the plastron, and within the plastron although only for short periods. Here we developed methods to continuously measure plastron PO₂, and investigate how it is affected by temperature (15, 20, 25°C), activity, and water circulation. We also made measurements of water PO₂, temperature and velocity in the field and swimming velocity at the treatment temperatures. Results show that plastron PO₂ is inversely related to temperature, associated with differences in metabolic demand, and that small bouts of activity or changes in water convection result in rapid changes in plastron PO₂. A model was developed to calculate the conditions under which Aphelocheirus would exist without becoming O₂-limited in relation to water temperature, PO₂ and boundary layer thickness. This suggests that Aphelocheirus at one of two field sites may have a reduced metabolic scope even in well convected water in association with low PO₂ and moderate temperature, and that in well convected, air-saturated water, bugs may have a reduced metabolic scope where water temperatures are between 20 and 25°C. If exposed to 5kPa PO₂, Aphelocheirus cannot sustain resting metabolic rate even in well-convected water and would die at temperatures above approximately 25°C.

Superhydrophobic hierarchically structured surfaces in biology: evolution, structural principles and biomimetic applications

A comprehensive survey of the construction principles and occurrences of superhydrophobic surfaces in plants, animals and other organisms is provided and is based on our own scanning electron microscopic examinations of almost 20 000 different species and the existing literature. Properties such as self-cleaning (lotus effect), fluid drag reduction (Salvinia effect) and the introduction of new functions (air layers as sensory systems) are described and biomimetic applications are discussed: self-cleaning is established, drag reduction becomes increasingly important, and novel air-retaining grid technology is introduced. Surprisingly, no evidence for lasting superhydrophobicity in non-biological surfaces exists (except technical materials). Phylogenetic trees indicate that superhydrophobicity evolved as a consequence of the conquest of land about 450 million years ago and may be a key innovation in the evolution of terrestrial life. The approximate 10 million extant species exhibit a stunning diversity of materials and structures, many of which are formed by self-assembly, and are solely based on a limited number of molecules. A short historical survey shows that bionics (today often called biomimetics) dates back more than 100 years. Statistical data illustrate that the interest in biomimetic surfaces is much younger still. Superhydrophobicity caught the attention of scientists only after the extreme superhydrophobicity of lotus leaves was published in 1997. Regrettably, parabionic products play an increasing role in marketing.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.