1. The levels of potassium, sodium, magnesium and calcium in leaves, midgut contents, midgut tissue, and blood were analysed in seven developmental stages between feeding, fourth-instar larvae and new pupae of the Cecropia silkworm. 2. Three dramatic changes in cation levels were found: the K level in the contents drops from 284 /+- 51 mEquiv./1 tissue water in the fifth-instar larva to 51 +/− 6 mEquiv./1 in the new pupa; the Mg level in the midgut tissue increases from 28 +/− 3 mEquiv./1 at the time of gut evacuation to 1093 +/− 104 mEquiv./1 in the new pupa; and the Ca level in the contents drops temporarily from 56 +/− 12 mEquiv./1 in the feeding fourth instar larva to 17 +/− 5 mEquiv./1 in the new fifth instar larva. The Na level was nerve higher than 2.8 +/− 0.5 mEquiv./1. 3. The relative levels of the four cations were different for each tissue studied, but each tissue maintained the same relative levels during the developmental stages studied. The sequences are: leaf, Ca greater than K GREATEr than Mg greater than Na; midgut contents, K greater than Ca greater than Mg greater than Na; midgut tissue, K GREATEr than Mg greater than Ca greater than Na; and blood, Mg greater than K greater than Ca greater than Na. 4. There were three large concentration gradients across the midgut; the K level in the midgut contents is approximately 10 times the level in blood; the Mg level in contents is one-half to one-sixth the level in blood; and the Ca level in contents is 3–4 times the level in blood. The K gradient and the Ca gradient are opposed and the Mg gradient is favoured by the electrical gradient across the larval midgut, the contents being 100 mV positive with respect to the blood. The K gradient and the electrical gradient are not present across the pupal midgut while the Mg gradient and the Ca gradient persist. 5. The K gradient is presumably maintained by the midgut K pump, the Mg gradient is aided by the midgut Mg pump, and the Ca gradient suggests that the midgut may possess a Ca pump.

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