The composition of larval-pupal moulting fluid in the tobacco hornworm, Manduca sexta

Abstract

The concentrations of sodium, potassium, chloride, bicarbonate, total phosphate, labile phosphate, inorganic phosphate, protein, polypeptides, amino acids, trehalose, and glucose, as well as pH and osmotic pressure of larval-pupal moulting fluid and haemolymph were measured in the tobacco hornworm, Manduca sexta, during the secretion and resorption of moulting fluid. Moulting fluid is a mildly alkaline (pH 7.2), iso-osmotic (330 mOsm) potassium bicarbonate salt solution containing within it the sol form of moulting gel. Bicarbonate is the principal anion in moulting fluid. It is only a minor component of haemolymph. Significant differences in the concentrations of potassium, chloride, bicarbonate, non-labile phosphate, labile phosphate, inorganic phosphate, protein, polypeptide, amino acids, trehalose, and glucose indicate that the integumentary epithelium does not act as a semi-permeable membrane, and that free diffusion between moulting fluid and haemolymph can account for only a small fraction of the molecular species movement between these two fluids.