The fine structure of the epithelial cells of the rectal papillae in the blowfly, Calliphora erythrocephala Meig., has been investigated to elucidate the possible mechanism of reabsorption of water and ions from the rectal lumen. To observe the variations in the structure of the epithelium in response to the absorptive processes the material was taken (a) from flies at the various stages of their first oviposition cycle, and (b) from freshly emerged imagoes starved for 2 days and injected into the rectum with solutions of various tonicities. It has been found that the complex system of intercellular spaces, formed by a prolific infolding of the lateral plasma membrane of the cells, shows a direct response to the conditions of supposedly maximal and minimal transport of fluid. These spaces are (a) grossly distended in the flies injected with hypotonic media, (b) highly dilated under normal conditions, and (c) completely collapsed in fasting and starved flies. These observations have been discussed in the light of the available theories to explain the mechanism of water transport in biological tissues. It is proposed that the structural design of the rectal papillae favours the application of double-membrane theory to explain the reabsorption of water against osmotic gradients as a consequence of an active transport of solutes into enclosed spaces.

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