AN ELECTRICAL MODEL FOR PERIPLANETA-AMERICANA PRONOTAL INTEGUMENT - AN EPIDERMAL LOCATION FOR HYDRATION-DEPENDENT RESISTANCE

Measurements of electrical resistance appear to be useful indicators o f porosity and related water permeability in insect cuticle, To develo p an adequate understanding of how such measurements relate to the phy sical and structural properties of the integument, we made detailed de terminations of resistance and impedance values for pronotal cuticle i n adult male Periplaneta americana, The most consistent estimates were obtained by averaging measurements across the integument on both side s of the midline at several intervals starting 15 min after electrode application. Pronotal resistance varied inversely with water content, from about 10 k Omega cm(2) in hydrated cockroaches to about 40 k Omeg a cm(2) in dehydrated insects, Though the dermal gland canals appear t o act as the main conductive pathway across the cuticle, the variable 'barrier' is located in the epidermal layer, since removal of the epid ermis from isolated pronota also removes most of the variable componen t of the integumental resistance, Comparison of measurements between t wo external electrodes with single-electrode measurements revealed a ' shunt' pathway parallel to the cuticle surface; modelling this shunt s uggested that it was variable and located mostly internal to the cutic le, supporting an epidermal location for a variable barrier, Impedance measurements over a range of frequencies showed the integument to be electrically complex, and a model is proposed to account for its prope rties