The effects of a fluctuating environment on the water relations of larval Lepidoptera

Abstract. New techniques for haemolymph analysis under field conditions have permitted studies of short-term changes in the body fluid concentrations of insects, in association with ambient conditions.
The haemolymph of four species of caterpillar changes in relation to the prevailing relative humidity, but the magnitude of these changes was correlated with the degree of exposure to the environment characteristic of each species and instar. In larvae habitually fully exposed to ambient conditions, haemolymph concentrations normally changed predictably through a day in relation to the humidity. However, osmoregulatory mechanisms operated to keep the increments in osmolality within acceptable limits. For those larvae which inhabit protected microenvironments with constant high humidity, either in association with plants or in conspecific aggregations, ambient humidity had little effect on the blood when this was sampled in situ. But when removed from such habitats, blood osmolality increases were large and rapid, and the larvae often succumbed to desiccation.
The role of biochemical, physiological, behavioural and ecological strategies in establishing the water balance of an insect in its natural surroundings are considered in the light of these findings.