Thermoregulation and daily activity patterns in a black desert grasshopper, Taeniopoda eques

Abstract

Behavioural thermoregulation was studied in the western horse lubber grasshopper Taeniopoda eques (Burmeister), a native of the Chihuahuan Desert of North America. The grasshoppers regulated their temperature through a series of daily cyclical vertical movements between vegetation and the soil, and by the adoption of four thermoregulatory postures: flanking, crouching, stilting and stem-shading. At dawn, the grasshoppers moved from their nocturnal roost-plants to the ground, returned to bushes during the middle of the day, moved back to the open ground in the afternoon, then reascended vegetation at dusk. The occurrences of the four thermoregulatory postures were synchronized with these microhabitat shifts. During the cooler mornings and afternoons, the insects maximized heat gain by flanking and crouching, achieving thoracic temperatures of up to 16°C above ambient. Throughout the hot middle of the day the insects stilted and shaded, minimizing heat gain. These behaviours effectively kept the grasshoppers' body temperatures near the preferred temperature (36·2°C), but lower than the maximum voluntarily tolerated temperature (41·9°C), critical thermal maximum (45·2°C) and instantaneous lethal maximum (46·5°C). The body size of flanking insects influenced heating and cooling rates, wind effects and temperature excess at equilibrium. Both infrared and visible radiation appeared to elicit flanking. The need and ability to thermoregulate are influenced by this insect's reliance on chemical deterrents for defence.