The porous plates of coniform chloride cells in mayfly larvae were studied by electron microscopy using carbon replicas, whole-mount cuticles and thin sections. The cuticular skeleton of the porous plates is a local differentiation of the epicuticle forming a structure of extremely high organization. It is composed of cylindrical rods, which measure 20 nm in diameter, and are oriented perpendicular to the cuticular surface. They form an overlapping hexagonal pattern and are interconnected by double rows of transverse bars which are arranged in a complicated but regular fashion. This spongy skeleton defines a continuous space system which may be characterized as composed of triangular pores, the sides of which measure 20 nm. The pores are arranged in a hexagonal pattern, and interconnected via slit-like communications. The porous plates are covered with a porous lamina which possesses 2·5-nm-wide pores in hexagonal arrangement.

Results obtained from negative staining, infiltration with colloidal lanthanum hydroxide and histochemical precipitation of sodium indicate that the porous system provides pathways for solutes across the cuticle.

The 3-dimensional architecture of the porous plates is reconstructed in a model that also includes morphogenetic aspects.

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