Fluid movements during water-vapour absorption by the desert burrowing cockroach, Arenivaga investigata

The desert cockroach supplements its water intake by condensing atmospheric water onto the surface of protruded hypopharyngeal bladders. An associated pair of structures, the frontal bodies, produce a fluid which is conveyed to the bladder surface and added to by condensation. Reduced water activities on the bladder surface are maintained apparently by continuous removal of frontal-body fluid and condensate. This removal was demonstrated through application of nl quantities of fluorescent and radioactive tracer solutions to the bladder surface. Cuticular hairs are arranged on the bladder so as to divide it into a small (< 60 pl) suprasurface compartment and a large (32 nl) subsurface compartment. Rapid disappearance of fluorescent tracer from the bladder surface indicated movement of fluid from the suprasurface to the subsurface compartment. Radioactive tracer solutions were removed from the bladder, traversed the posterior hypopharynx, and accumulated in the oesophagus and crop. It is suggested that condensed fluid is conveyed to the crop in the same way. Tracer solutions were restricted to cuticular surfaces of the bladders, posterior hypopharynx, oesophagus and crop; other mouthparts are not involved in fluid movements. Movement of fluid into the subsurface compartment, and the small proportion of tracer solution which could be recovered from the bladder by Millipore filters with a pore size of 0.1 μm suggested the maintenance of considerable surface tensions on the bladder surface. Rapid removal of condensed fluid from the suprasurface compartment, where fluid is exposed to the atmosphere, suggests that the absorption mechanism is not dependent upon the properties of a hygroscopic solution. Over 99% of such a solution would be in the subsurface compartment, not exposed to the atmosphere, and would therefore be unavailable for the reduction of water activity.