The pigmy backswimmer, Plea minutissima, from time to time leaves the water to apply its antimicrobial metathoracic gland secretion to its water-repellent ventral pubescence (“secretion-grooming”) which is enclosed by an air sheath when submerged and has a respiratory function. Secretion-grooming keeps the hairs free of microbial contamination and thus hydrophobous and functional. The grooming behaviour is regulated by abiotic factors. It is released by an increase in light intensity or water temperature, with a sudden rise in temperature being particularly effective. The higher the temperature and light intensity, the faster the grooming act is performed and completed. The grooming motivation is higher at a higher ambient temperature, but declines once the behaviour has been brought to completion. As a consequence the highest grooming activity in the field is observed in summer on the first sunny day after a prolonged period of overcast days. The tying of secretion-grooming to abiotic factors probably serves to destroy epizoic microorganisms before they begin to multiply more rapidly at higher temperatures. The mechanisms regulating the secretion-grooming behaviour of Plea may be of significance also for the regulation of certain other insect behaviours (e.g., flight behaviour).