Tobacco hornworm (Manduca sexta) caterpillars are able to regulate the water content of their body when fed on diets of markedly different water content. This regulation extends to the water content of food within the gut. Regulation of body water is achieved by adjusting the amounts of water lost with the faeces. The rectum is shown to be the principal site of water reabsorption from the faeces. The rate of rectal water absorption is shown to vary with the water content of the food and thus according to need. Water reabsorbed from the rectal contents is recycled and added to the contents of the midgut. The ultrastructural appearance of epithelial cells in the rectal wall is that expected of a fluid-transporting tissue. The ileum appears to play little or no part in water recycling.

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