Respiration and water loss

When being exposed to relatively dry air, insects rapidly lose their body water via the skin (cuticle) or via open spiracles when breathing (respiratory water loss). Water loss can be an important stress factor or lethal when water supply is limited. We compare metabolic rates and water loss across many different insects from different environments, trying to understand the factors that contribute to the variation in water loss and metabolism: body size, surface/volume ratio and the protective epicuticular wax layer.

For water loss measurements, Jule Mangels, Karl Schuller and Manuel Carmago delevloped a much finer apparatus that continuously monitors the insect's water loss - measuring the increase in humidity caused by the insect in a chamber with controlled air flow. In some species we can even distinguish brief periods of increased water loss when an insect opens its spiracles (discontinuous gas exchange). The data are continuously monitored for ten chambers that are simultaneously used. 

For measuring metabolic rates, we use a micro-respirometer by Stefan Scheu (1992), which was rebuilt and refined by Karl Schuller. Insects (or other probes e.g. substrate with microbes) are placed into a glass chamber, where exhaled CO2 is bound to a KOH solution, reducing the inner pressure which is measured by a sensor. This induces an equivalent O2 release from an electrolytic chamber to restore the original pressure level. Sensor pulses are counted in 15 min intervals to monitor the respiration rate over time. The system is sensitive to measure respiration even for individual insects if they are not too small. The air temperature in the chambers is controlled and can be modified via a surrounding water bath. 

Reference for micro-respirometer

Scheu S (1992) Automated measurement of the respiratory response of soil microcompartments: active microbial biomass in earthworm faeces. Soil Biology and Biochemistry 24: 1113-8