Water and carbon dioxide loss from the cockroach Periplaneta americana (L.) measured using radioactive isotopes

Tritiated water (THO) loss from Periplaneta americana is used to measure cuticular permeability (Pd). In dry air, following various periods of predesiccation, the values of Pd lie around 0.5 x 10(-9) ms-1, with the lowest values below 0.3 x 10(-9) ms-1, close to the lowest permeabilities previously reported. There is no obvious relationship between permeability and initial mass of the insect. A significant lowering of Pd was found after 72 h of predesiccation. A marked sudden decrease in permeability occurred when the airstream flowing over an insect was changed from humid air to dry air. Labelling the bicarbonate pool using 14CO3(2-) enabled the output of CO2 to be estimated. An average output of about 390 microliters g-1 h-1 was found. Simultaneous measurement of THO and 14CO2 loss showed that tracheal water loss is only a small component of total water loss, with an average value of 3.8%. The total water loss can therefore be used to make close estimates of cuticular permeability in Periplaneta americana. 2min sampling periods clearly show the pulsatile nature of 14CO2 release but, even when the effect is most discernible, the periodic release of CO2 has only a small effect on water loss.