Function and Evolution of Thermoregulation in the Desert Grasshopper Taeniopoda eques

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Abstract
(1) For Taeniopoda eques, most essential activities are influenced by body temperature: flying, walking, jumping, morning descent, feeding, defecation, molting, stridulation, mating, and oviposition rates are directly temperature-dependent. Furthermore, survivorship and developmental rates, and ultimate size and weight vary with rearing temperature. By behaviourally thermoregulating, T. eques maintain their temperature between 30 and 40.degree.C for much of the day, and thus optimize its effects. (2) T. eques survives in a desert environment with a short and unpredictable favourable season that averages 692 degree-days (DD). However, this grasshopper requires 850 DD to develop. By thermoregulating, T. eques increases its developmental rate, enabling it to complete its life cycle before the habitat fails. In this sense T. eques is a ''maxitherm''. (3) Critical to the understanding of the evolution of thermoregulation is the relationship between enzyme activity rates and temperature: enzymes function best within certain narrow temperature ranges. For optimal physiological performance, an animal''s body temperature must be kept within these limits. For ectotherms, this range must reflect the highest environmental temperature that the population normally encounters. (4) The ability and need to thermoregulate are influenced by non-thermal features such as defensive strategy, body size, and feeding strategy. For this grasshopper, the antipredation features of black aposematic coloration and unpalatability allow rapid radiative heating, and extended conspicuous exposure. Release from vertebrate, but not invertebrate, predation favours large size, a feature beneficial for water retention and deep oviposition. Size also influences thermal inertia, and ultimate temperature excess during solar heating. A high thermal inertia is detrimental for T. eques, considering the short period available for morning foraging. The need to ground forage thus makes thermoregulation essential.

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