An important function of the rectum in terrestrial insects is the removal of water from the faeces, which in some insects are in equilibrium with an atmosphere of 75% relative humidity or less when they are voided. Structural complexity in the insect rectum appears to have evolved along two main lines: ( a ) The cryptonephric line; the ends of the Malpighian tubules are enclosed together with the rectum in a perirectal space. ( b ) The rectal gland line; areas of thickened epithelium have a system of intercellular channels and spaces. These two lines have in common that the perirectal fluid of ( a ) and the intercellular fluid of ( b ) are of higher osmolarity than the haemolymph, and it has been suggested that the reabsorption of water from the faeces is thereby facilitated. Possible mechanisms are discussed, with special reference to the active transport of water.