The Permeability of the Cuticular Lining of the Insect Alimentary Canal

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Abstract
The permeability of the cuticle lining the foregut and hindgut of adults of Schistocerca gregaria, Gromphadorhina portentosa, and Leucophaea maderae, and larvae of Manduca sexta was investigated. In Schistocerca, Manduca, Leucophaea, and Gromphadorhina the cuticle lining the foregut has a very low permeability, while that lining the ileum (Schistocerca, Manduca, Leucophaea, and Gromphadorhina) and the rectum (Schistocerca and Manduca) is much more permeable. The low permeability of cuticle lining the crop in the insects examined is suited to the crop's function as a temporary store of ingested material. In Schistocerca gregaria, the cuticular lining of the ileum and rectum were found to be equally permeable. Each allows relatively rapid passage of small neutrally charged hydrophilic molecules. Both, however, showed a reduced permeability to anions and an enhanced permeability to cations. The cuticular lining of the hindgut would thus not be a barrier to the reabsorption of useful substances known to appear in the fluid reaching the hindgut from the Malpighian tubules, but it would, to some extent, retain in the lumen the relatively large anions actively excreted by the Malpighian tubules. The cuticle lining the colon of Schistocerca gregaria has a much lower permeability than has that lining the rest of the hindgut. This accords with the idea that the colon is thought not to be involved in absorption.