 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 ) . 
