Transpiration and humidity preference in a temperate wood ant Formica rufa L.

Abstract

Elevation of ambient temperature resulted in an increase in the rate of water loss from worker ants, greatest loss occurring in the first hour. Workers exposed to high humidities (> 75% r.h.) absorbed water onto the cuticle. There was no correlation between body size and transpiration rate below 40°C except in the case of callow workers which showed a significant correlation at 25°C. There was evidence for a physical change in the cuticle at 40°C producing an acceleration in the rate of transpiration. The mean total body water content of workers was calculated as 77.2 ± 6.2%. Mortality was approx. 50% when total body water fell to 46%. Workers were found to be slightly more permeable to water than males and queens. Callow workers lost water at twice the rate as mature workers. Pupae were highly resistant to desiccation. Seasonal effects on the rate of transpiration could not be detected in workers exposed to temperatures below 40°C. Photoperiod was found to influence the choice of humidity by workers in a gradient apparatus. Under 16 h light-8 h dark, workers showed preference for the highest humidity; on 10 h light-14 h dark, they appeared indifferent. Callows chose the highest humidity (95% r.h.). Formica rufa are probably tolerant of dry conditions sometimes experienced in temperate environments, photoperiod being important in the control of water balance.