The alkali fly, Ephydra hians Say, inhabits alkaline salt lakes which can contain concentrations of dissolved carbonate and bicarbonate as high as 500 mmol l−1. Larvae of the alkali fly possess two pairs of Malpighian tubules. The posterior pair has a morphology similar to that of the tubules of most other insects, but the anterior pair is modified into an enlarged gland containing white microsphere concretions. We describe the ultrastructure of all cell types in both pairs of tubules. Using scanning electron microscope (SEM) X-ray microanalysis and chemical CO2 quantification, we demonstrate that the concretions in the lime glands are composed of nearly pure calcium carbonate. Isolated preparations of lime gland tubules accumulate 45Ca significantly more rapidly than do normal tubules. Although similar to the rime concretions found in the Malpighian tubules of other Diptera, the lime glands of this insect may function to regulate the high concentrations of carbonate and bicarbonate encountered in their aquatic environment. It is proposed that the mechanism of this regulation may be chemical precipitation of carbonate/bicarbonate with calcium in the lumen of these specialized lime gland tubules.

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