The midgut of the American silkworm transports potassium actively from the blood-side to the lumen (Harvey & Nedergaard, 1964) and when the midgut is short-circuited, the short-circuit current (ISC) is approximately equivalent to the active K transport. The rate of movement of water has been considered to be low. It has been measured as 9 ±70μl h−1 (N=3) from blood-side to lumen in an open system (Nedergaard, 1972). In the closed system described by Harvey, Haskell & Zerahn (1967) the water flow could be monitored from the shape and size of the midgut and it was obvious that the movement of water was small.

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