Abstract
Hormonal control of fluid reabsorption in the rectum is a major mechanism of water balance homeostasis in terrestrial insects. We report that steadyβstate fluid transport in the absence of, or against, an osmotic gradient can be driven by absorption of Na^+^, K^+^, or Cl^β^ from the lumen. Therefore chloride transport stimulating hormone (CTSH), a newly discovered hormone which stimulates KCl reabsorption by elevating tissue cAMP, might enhance fluid absorption indirectly by its action on transport of the major solute. This prediction was not confirmed and explanations are discussed. Rectal tissue volume increases considerably during the first hour in vitro and this epithelia only switches from slow transport of primarily NaCl to rapid absorption to predominantly KCI (the in vivo situation) after stimulation. Metabolic support of KCl transport depends largely on proline absorption from Malpighian tuble fluid entering the rectum. Since these specific requirements and properties of in vitro rectal preparations were not known during earlier investigations, most reports of putative diuretic and antidiuretic factors acting on rectal fluid transport in vitro require reexamination and confirmation.