Thermal Physiology of Cicadas and Regulation of Cuticular Permeability in Insects

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Dr. Toolson has recently discovered that cicadas can reduce their body temperature by evaporation of water. This ability is known only for a handful of insects. What makes cicadas unique among all animals except mammals is that they achieve the high water loss rates required for evaporative cooling by means of an energy-dependent process that causes extrusion of water through large-diameter cuticular ducts. The extrusion of water is triggered when a cicada's body temperature threatens to go above a specific setpoint (Tsp), and is thus the functional equivalent of sweating in mammals. The proposed research entails two somewhat distinct themes. First, extending the PI's discovery that cicadas can evaporatively cool by activating an energy- dependent process that facilitates transcuticular water flux (TWF) rates, the PI's will conduct studies to elucidate the chemical and neurological mechanisms involved in regulation of the TWF- facilitating process. Compounds, such as aspirin, acetominiphen, and prostaglandins, that have been shown to affect body-temperature regulation in mammals, and compounds with known regulatory activity in insects, e.g., octopamine, will be tested for their effect on TWF and, therefore, on body temperature. Localized regions of the cuticle will be warmed to determine if this affects TWF. Classic ligation experiments will be used to assess whether the nervous system regulates TWF and, if so, at what level neurological regulation is mediated. Finally, acclimation experiments and comparative studies involving cicada species from a variety of habitats will be conducted to elucidate the interaction of thermal regime, size, color/reflectivity, and other factors in determining Tsp. The second part of the proposed research consists of experiments designed to test between two alternative hypotheses proposed to account for the habitat-associated and seasonal patterns of cuticular permeabilities reported for most terrestrial arthropods. Cockroaches, crickets, and scorpions will be subjected to different combinations of food and water availability to determine whether the ability to modify cuticular permeability in response to changing desiccation stress reflects a need to conserve energy/precursors or to regulate rates of TWF.