The presence of a cation pump on the apical membrane of epithelial cells in insects, responsible for the extrusion of K or Na, has been proposed for Malpighian tubules (Maddrell, 1977) and salivary glands (Berridge, Lindley & Prince, 1976). An essential role of the midgut in K homeostasis has been established only for the phytophagous larvae of Lepidoptera (Harvey & Nedergaard, 1964). The main features of the rheogenic, luminally-directed potassium pump of this tissue have been extensively studied (Zerahn, 1978; Harvey, Cioffi, Dow & Wolfersberger, 1983). Since the midgut of these animals is also peculiar in that it does not actively absorb Na, the presence of the K pump could be the sole example in insect gut physiology. Na absorption in the midgut has been demonstrated in several species of insects (O’Riordan, 1969; Sauer, Schlenz True & Mills, 1969; Farmer, Maddrell & Spring, 1981 ; Dow, 1981a; Koefoed & Zerahn, 1982) while K secretion has been suggested for Periplaneta americana (Sauer & Mills, 1969), Leucophaea maderae (Sacchi & Giordana, 1979) and Schistocerca gregaria (Dow, 1981b).

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