On the mechanism of water vapour sorption from unsaturated atmospheres by ticks

Abstract

The nascent salivary secretion of 41 partly dehydrated and unfed adult female Amblyomma variegatum, 5­8 months post-ecdysis, during water vapour uptake at 93.5 % relative humidity and 20 °C, had an osmolality of between 298.6 and 769.7 mosmol kg-1 (mean ± s.d. 470.3±85.8 mosmol kg-1). This range would allow water vapour uptake at relative humidities of approximately 98­99 %, but it would not suffice for lower relative humidities down to 80­85 %, the critical equilibrium humidity of A. variegatum. At this relative humidity (85 %), an osmolality of 9796 mosmol kg-1 is required for water vapour uptake. It is proposed that hydrophilic cuticle in the hypostome could play a role in water condensation and that the slightly hyperosmotic secretion of the agranular alveoli of the salivary glands might alter the water affinity at the adsorbing cuticle surface and release the adsorbed water. The water-enriched secretion would then be drawn into the mouth by the powerful suction of the pharynx. This hypothetical hydrophilic cuticle component of water vapour uptake in A. variegatum merits closer investigation. The sorption kinetics of A. variegatum support an additional 'osmotic' component of water vapour uptake at humidities near saturation. A nanolitre osmometer particularly suited to sample volumes smaller than 5 nl was developed. This device does not require the transfer of fluid after collection, and its measurement range is extended beyond the 5 osmol kg-1 that can be measured using commercial apparatus.