Evaporative cooling in ectothermic cicadas

Behavioural thermoregulation in cicadas is interesting in that it may be supplemented with physiological mechanisms at the different extremes of Ta: evaporative cooling and endothermy. Evaporative cooling, long assumed to play a minor role in the heat balance of insects because of their small water reserves, is the only means by which insects can reduce Tb below Ta. Exceptions do occur in hot environments, such as the evaporative cooling which occurs by cuticular and respiratory routes in cicadas and grasshoppers, respectively (Prange 1990; Toolson 1987) and by an oral route in honeybees (Section 6.5.3). With an abundant source of water from plant xylem, desert cicadas are able to sing during midday heat that their enemies can not tolerate. Hadley et al. (1991) used a flow-through system to measure body temperature, water loss, and gas exchange simultaneously in a Sonoran Desert cicada, Diceroprocta apache, while it was feeding on perfused twigs. Evaporative losses increased dramatically above Ta of 38°C, and higher body water contents in the cicadas were associated with greater depression of Tb below Ta. The 13-year periodical cicada, Magicicada tredecem, shows limited cooling ability, which may represent the ancestral condition in cicadas (which are apparently tropical in origin), while the more efficient cooling mechanism of D. apache may be a derived evolutionary response which works well in low humidities (Toolson and Toolson 1991) (see also Section 4.1.1). Endothermy in cicadas is associated less with flight than with reproductive behaviour, and enables male cicadas to sing in the rain or after sunset, when bird predators are inactive (Sanborn 2000; Sanborn et al. 1995). Singing involves intense activity of the tymbal muscles in the first abdominal segment (Josephson and Young 1979), unlike stridulation in katydids which depends on flight muscles (Stevens and Josephson 1977).

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