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 (P_d). In dry air, following various periods of predesiccation, the values of P_d lie around 0.5×10⁻⁹ m s^-1, with the lowest values below 0.3×10⁻⁹ m s^-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 P_d 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 μl 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. 2 min 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.