• Ectotherm;
  • fitness;
  • generalist;
  • specialist

1. Ectotherms may be thermal generalists, or high- or low-temperature thermal specialists. The thermal strategy of four species of grasshoppers occurring in Britain is determined, where unpredictable variation in the generally cool climate should preclude the low-temperature thermal specialist strategy. It is predicted that temperature sensitivity will determine geographical distribution, with generalist species widespread, and thermally specialized species restricted to warmer habitats.

2. The developmental and reproductive responses to different rearing temperatures of the grasshoppers are examined in a laboratory experiment. Life-history traits are integrated into a fitness model to determine the sensitivity of each species to temperature change.

3. Growth and development rates increased with temperature for each species. The frequency with which an additional instar was inserted during nymphal development increased with temperature in Chorthippus brunneus. Adult mass and size increased with temperature.

4. Egg pod production rate increased with temperature. In Omocestus viridulus, Myrmeleotettix maculatus and Stenobothrus lineatus, temperature had no effect on egg mass, eggs per pod or number of pods per female. Number of pods per female increased with temperature in C. brunneus.

5. Fitness of S. lineatus decreased by 88% for a 5 °C fall in temperature compared with 58% and 56% for C. brunneus and M. maculatus, respectively. Omocestus viridulus is least sensitive to temperature change with only a 27% reduction in fitness at the lower rearing temperature.

6. It is concluded that all the species are high-temperature thermal specialists, and variation in their sensitivity to temperature is a good predictor of their distribution. The most generalist species, O. viridulus, is the most widespread, while the more specialist species S. lineatus and M. maculatus are restricted to warmer habitats. Chorthippus brunneus is also a high-temperature specialist, but is more widespread as a consequence of developmental and reproductive plasticity and efficient behavioural thermoregulation.