We describe a model which estimates grasshopper body temperature (Tb) by linking energy-flow equations with empirical descriptions of aboveground gradients of air temperature (Ta) and wind speed. The model was tested using restrained grasshopper nymphs; estimated and observed Tb agreed well (r2 > 0.81). At a rangeland site near Lethbridge, Alberta, Canada (49 °42′N, 112 °48′W), we observed 315 free-living grasshoppers. We recorded the shadow each cast on a horizontal surface, then reconstructed their orientation to the sun by geometric analysis. We used the model to estimate their Tb and the range and frequency of possible Tb within their environment. Modelled Tb exceeded Ta, and was generally lower than the modelled maximum possible Tb, but was well correlated with Tb of insects on top of the dense layer of vegetation which pervaded the site. This observation suggests that behaviours which elevate Tb are constrained by environmental barriers. Tb exceeded the value expected if insects were located and oriented randomly within their environment (mean difference = 3.95 °C, SE = 0.115); this is unequivocal evidence for behavioural thermoregulation. Heuristic simulations using temperature-dependent developmental- and feeding-rate equations for Melanoplus sanguinipes (Fabricius) suggest that thermoregulatory behaviour increased these rates by 30–40% compared with those for insects located and oriented randomly within their environment. During this study, population processes were never inhibited by excess heat; therefore any climatic warming at the experimental site will probably accelerate the phenology of these grasshopper species. Effects at other sites may differ; the model can be applied to test this possibility.

Feeding rates of Colorado potato beetle larvae were measured in 24-h trials in small field cases on potato leaflets that remained attached to the plants. Four models were constructed to explain the results; these differed by including or excluding the effect of insolative heating on larval body temperature, and by including or excluding thermoregulatory behavior, in all combinations. In all instars, observed feeding was independent of mean body temperature during the trial as estimated by each model, and less than the amount predicted using this mean to drive constant-temperature feeding rate functions.

The four models were assessed by regressing model predictions on observed feeding rates. For first and second instars, there was no relationship between observed feeding and the amount predicted by any of the models, probably because the feeding rate was small relative to the precision of the measurements. For third and fourth instars, predictions from the model that included both insolative heating and behavioral thermoregulation did not differ significantly from observations. Simulations using this model suggested that daily mean body temperature fails to predict foliage consumption because this mean cannot account for the effects of transient occurrences of suboptimal temperatures.