 Bumblebees are active in conditions when honeybees stay at home, and can readily absorb heat from even weak sunshine. The thick pile created by long setae (bristles) acts as insulation to keep bumblebees warm in cold weather; species from cold climates have longer setae (and thus thicker insulation) than those from the tropics. The temperature of the flight muscles, which occupy much of the thorax, needs to be at least 30°C (86°F) before flight can take place. The muscle temperature can be raised by shivering. It takes about five minutes for the muscles to reach this temperature at an air temperature of 13°C (55°F).
