1. Carbonic anhydrase was measured in tissues of silkmoths, Hyalophora cecropia, reared on either a wheatgerm-based synthetic diet or wild cherry foliage in feeding fifth-instar larvae, throughout the larval舑pupal transformation and in newly ecdysed pupae.

2. Carbonic anhydrase activity was present in fat body, midgut and intgeumentary epithelial cells, but not in haemolymph, cuticle or the integumentary musculature.

3. Approximately 70 % of the total carbonic anhydrase present per animal is localized within the epidermal cells of the integument.

4. The midgut is without measurable carbonic anhydrase activity from the day of apolysis (through the newly ecdysed pupal stage in development) until after the larval-pupal ecdysis.

5. In tissues analysed during the feeding fifth larval instar, 150 toward acetazolamide was between 10舑8 and 10舑7 m.

6. With the exception of larval midgut, regardless of stage, tissue or diet, both potassium and chloride normally inhibit carbonic anhydrase, with the effects of potassium and chloride being additive.

7. In larval midgut, chloride at concentrations of 50 or 150 mm stimulates carbonic anhydrase activity 30 or 100%, respectively.

8. In foliage-reared insects, potassium enhances chloride stimulation of larval midgut carbonic anhydrase, whereas in synthetic-diet-reared insects, potassium antagonizes the stimulatory effects of chloride.

9. Removal of a heat-stable, dialysable factor associated with the larval midgut of foliage-reared insects converts properties of the foliage midgut enzyme to those characteristic of the synthetic diet type, and vice versa.

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