The development of polarity in a simple epithelium, the Malpighian tubules of Rhodnius, is analysed both ultrastructurally and physiologically. The onset of physiological function, including fluid secretion and the transport of solutes, is determined in late embryos and young hatchling insects and compared with structural development in tubules over a similar period. Two phases of maturation, separated by several days, are detected. The first, during late embryogenesis, involves the development of mature intercellular contacts and the dilation of the lumen, and is associated with the ability to transport specific solutes. The second phase involves the elaboration of the apical and basal membranes and the generation of mitochondria, and is associated with the onset of fluid transport in the tubules and with feeding in 4-day-old hatchlings.

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