Hyperosmotic oral fluid secretion during active water vapour absorption and during desiccation-induced storage-excretion by the unfed female tick Amblyomma americanum

It is well established that some acarines, insects and isopods maintain water balance in subsaturated air by actively absorbing water from the atmosphere (O’Donnell and Machin, 1988). Investigations of water balance physiology by Lees (1946) in the Ixodoidea demonstrated that partially desiccated ticks gain weight in water vapour activities (av=relative humidity/100) significantly lower than that of the haemolymph activities (av≥0.98). Subsequent research confirmed that active water vapour uptake is a common phenomenon in ticks (for reviews, see Rudolph and Knülle, 1978; Needham and Teel, 1986). The gnathosomal region was identified as the site of water vapour uptake when Rudolph and Knülle (1974) blocked various body surfaces of predesiccated adult Amblyomma variegatum with paraffin wax and monitored weight changes during exposure to a subsaturated av. A net weight gain occurred except when the gnathosoma was occluded. The external gnathosoma of the ixodid tick projects anteriorly from the basis capitulum as an assemblage of five processes (Fig. 1A). The most lateral appendages are a pair of palps that can be abducted from the inner dorsal chelicerae during attachment and blood feeding. The paired burrowing chelicerae and a singular ventral hypostome secure the oral sucking apparatus into the host feeding lesion during haematophagy. Wax-occlusion techniques have identified the proximal hypostome-cheliceral junction as a more specific site for vapour uptake in A. americanum (L.); and a correlation was found between palp abduction splaying behaviour and the critical equilibrium activity at which ticks were held (CEA) (M. D. Sigal, in preparation).