The basic morphology and ultrastructure of the anal papillae of a saline water mosquito larva (Aedes campestris) have been investigated under two physiological conditions: (a) normal hyperosmotic external medium and (b) dilute hyposmotic medium. The ultrastructure of these organs suggests that they are not rudimentary, but rather are made up of cells that are morphologically specialized for transport. The anal papillae appear active in both the normal and dilute media and possible functions

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... r the two physiological conditions are discussed. No major qualitative or quantitative differences were observed with the large change in external medium. This suggests that physiologically-demonstrated adaptive changes may require only minor structural alterations. The morphology and ultrastructure of the hind gut of A. campestris larvae were compared under two physiological conditions; normal (requiring hyporegulation) and dilute (requiring hyperegulation) external medium. No differences which might be associated with hyporegulation were observed. The rectum of A. campestri s was compared to that of Aedes aegypti. The rectum in the former is composed of two regions, an anterior and posterior rectum, while in Aedes aegypti, an exclusively freshwater mosquito larva, the rectum has only a single region. The rectal epithelia in both insects studied have morphological specializations for water and ion transport, and a consideration of quantative differences suggests that the posterior rectum is unique to Aedes campestris and hence could be responsible for the ability of this species to produce hyperosmotic urine. When the posterior rectal epithelium is compared to similar transporting epithelia in previously-studied terrestrial insects, several ultra-structural differences were noted. It is suggested that the mechanism of hyperosmotic urine production in saline water insects may be different from that in terrestrial insects. Several possible mechanisms of hyperosmotic urine production are discussed.