Mosquito larvae were raised to fourth instar in distilled water in order to maximally stimulate the salt-absorbing function of the anal papillae. Two exceptional features are observed, at the fine structure level, in the epithelial lining of the papillae. At the basal (cuticular) surface of the cells, the cell membrane is thrown into deep, narrow, parallel folds. The folds not only follow a rigid pattern at the cellular level but are also arranged at right angles to the long axis of the entire organ. A complicated pattern of canaliculi connects to the distal (plasma) surface of the cells. At this surface, paired mitochondria (sometimes triplets) are clamped about the membranes of the canaliculi to form structures referred to as "mitochondrial pumps." Mitochondria are also oriented in rather precise relation to the basal folds. Glycogen granules are found throughout the cytoplasm. The endoplasmic reticulum is sparse. The Golgi zones are flew and not well developed. Unidentifiable, irregular vesicles with lipid-like membranes are found associated with the basal folds. The distal (plasma) surfaces of the cells are covered by a homogeneous granular layer the composition of which is unknown.

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