Physiology and composition of molting fluid and midgut lumenal contents in the silkmothHyalophora cecropia

Abstract

1. The concentrations of sodium, potassium, calcium, magnesium, chloride, trehalose, total phosphates, labile phosphates, amino acids, proteins and polypeptides, and the osmotic pressures were determined in pharate pupal hemolymph and molting fluid, and pupal hemolymph and midgut lumenal contents. 2. The osmotic pressures of molting fluid and midgut lumenal contents were 40 and 56% greater, respectively, than those of hemolymph. To account for this increase in molting fluid, I proposed that cells in the epidermis actively transport potassium into the exuvial space with an accompanying passive bulk flow of water. 3. With the approach of ecdysis, the osmotic pressure of molting fluid remained constant, while the concentrations of organic components in molting fluid declined and those in hemolymph were simultaneously enhanced. 4. Differences in the levels of potassium, magnesium, labile phosphate and all organic components were so great as to preclude passive diffusion between molting fluid and hemolymph. 5. Fluid in the pupal midgut lumen appears to be derived primarily from the sloughed larval midgut epithelia and actively transported pigment derived from the larval integument. 6. Chloride, labile phosphate and amino acids were the major anionic components of hemolymph, while for molting fluid, they constituted less than 9% of the anions present. 7. The origins of fluids accumulating in the exuvial space of the cuticle and the lumen of the midgut are different, and both fluids are isolated from hemolymph.