Abstract
Tenebrionid beetle larvae survive prolonged dehydration as well as rapid water vapor intake without exchanging solutes with their environments. Tissue water is partially regulated at the expense of the hemolymph. This presumably optimum sharing of water is brought about by the mobilization of osmotically active solutes in hydrated animals and by their storage and osmotic inactivation in dehydrated specimens. The principal solutes involved are amino acids, trehalose, sodium, and chloride.
Between moults the rectal complex generates extremely high osmotic pressures (6.7 Osm.kg^β1^ in Tenebrio) for the purposes of fecal dehydration and water vapor absorption. Rectal osmotic pressures run down to hemolymph levels (about 0.6 Osm.kg^β1^) at the onset of moulting. Tubule and rectal epithelial cells show remarkably little change in volume over this osmotic range. Volume regulation probably protects the cells from bursting and increases the efficiency of osmotic coupling between rectal lumen and the tubules during water transport. The possibility of using ions from the gut to bring about regulation in epithelial cells is discussed.