LIFE-HISTORY RESPONSES OF BRITISH GRASSHOPPERS (ORTHOPTERA, ACRIDIDAE) TO TEMPERATURE-CHANGE

1. Ectotherms may be thermal generalists, or high-or low-temperature t hermal specialists. The thermal strategy of four species of grasshoppe rs occurring in Britain is determined, where unpredictable variation i n the generally cool climate should preclude the low-temperature therm al specialist strategy. It is predicted that temperature sensitivity w ill determine geographical distribution, with generalist species wides pread, 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 experim ent. Life-history traits are integrated into a fitness model to determ ine the sensitivity of each species to temperature change. 3. Growth a nd 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 i ncreased with temperature. In Omocestus viridulus, Myrmeleotettix macu latus 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 degrees C fall in temperature compared with 5 8% and 56% for C. brunneus and M. maculatus, respectively. Omocestus v iridulus is least sensitive to temperature change with only a 27% redu ction in fitness at the lower rearing temperature. 6. It is concluded that all the species are high-temperature thermal specialists, and var iation in their sensitivity to temperature is a good predictor of thei r distribution. The most generalist species, O. viridulus, is the most widespread, while the more specialist species S. lineatus and M. macu latus are restricted to warmer habitats, Chorthippus brunneus is also a high-temperature specialist, but is more widespread as a consequence of develop mental and reproductive plasticity and efficient behaviour al thermoregulation.