Symmetry and non-linearity of osmotic flow across rectal cuticle of the desert locust.

Abstract

ABSTRACT A method for measuring osmotic flow of water across rectal cuticle of the desert locust in vitro is described. The osmotic permeability coefficient decreases as the average osmolality of the two bathing solutions increases, probably due to dehydration of the cuticle. This is supported by a corresponding decrease in the diffusion permeability coefficient, as measured by flux of tritiated water in the absence of osmotic flow. Osmotic permeability is at least fifty times greater than diffusion permeability. This supports the hypothesis that osmotic flow occurs by bulk flow or single-file diffusion through pores. Isotopic flux measurements during net flow support this view. The osmotic permeability of the cuticle differs by less than 25% if at all, in the two directions of flow; that is, rectification or ‘valve-like’ behaviour is not significant. The cuticle represents only 2·6 to 13% of the total resistance to osmotic flow across the whole rectal wall and is not responsible for the asymmetry of the latter. Application of Beament’s model for water transport to the rectal wall of insects can be excluded in the case of the desert locust.