Measurement of water and carbon dioxide loss from insects using radioactive isotopes

A method is described for investigating the rate of loss of water and carbon dioxide from terrestrial insects by absorbing tritiated water and 14CO2 from a gas stream flowing past the insect. The loss of water and carbon dioxide can be studied simultaneously with a time resolution (nominal) down to 2 min. The theoretical and experimental bases of analysing the data are considered in detail. The determination of the efflux rate constant for water is straightforward and, if an estimate of surface area is available, the efflux rate constant can be converted to a permeability coefficient. In the case of 14CO2 loss, the interpretation is complicated by the presence of other compartments within the body that can be labelled with 14C. A multicompartment model of 14C exchange is developed and a method of obtaining the efflux rate constant of 14CO2 is described. The efflux rate constant for 14CO2 can be used to estimate CO2 output.