At the organismal level, desiccation resistance in insects is generally accomplished in three ways: by increasing body water content, by reducing rates of water loss, or by tolerating the loss of a greater proportion of body water (desiccation or dehydration tolerance) (Gibbs 1999). Each of these organismal traits is, in turn, an integrated measure of multiple physiological processes (e.g. water loss via the cuticle, respiration, or excretion). The costs of physiological regulation can be reduced by behavioural avoidance of desiccating conditions, as in insects with nocturnal or crepuscular activity patterns, or those which select favourable microclimates or form intraspecific aggregations (see below). Clear separation of behavioural and physiological regulation, however, is not realistic, as will become apparent from the examples to follow.
The standard method of measuring desiccation resistance is to record the mass change of insects maintained in dry conditions. Losses represent water and also dry matter metabolized as CO2, but in most studies respiratory water loss is assumed to be negligible. Useful water balance characteristics are the body water content, maximum tolerable water loss before death, time to maximum water loss, and rate of water loss. Survival time is equivalent to water loss tolerance divided by water loss rate, and is determined by initial water content. Note that insects maintained in dry air experience starvation and desiccation, whereas those maintained at higher av experience mainly starvation. Because initial differences in body mass may confound comparisons of absolute rates and tolerances (Packard and Boardman 1988), corrections for initial body mass are often used (e.g. Chown et al. 1999; Gibbs and Matzkin 2001).
Was this article helpful?