MECHANISMS OF THERMOREGULATION IN FLYING BEES

Thermoregulation of elevated thorax temperatures is necessary for bees to achieve the high rates of power production required for flight, an d is a key factor allowing them to occupy widely varying thermal envir onments. However, the mechanisms by which bees thermoregulate during E ight are poorly understood. Thermoregulation is accomplished by balanc ing heat gain and heat loss via the following routes: convection, evap oration, and metabolic heat production. There appears to be a diversit y of thermoregulatory mechanisms employed during Eight among bee speci es. Some species, particularly Bombus spp,, actively increase the dist ribution of thoracic heat to the abdomen during flight as air temperat ure (T-a) rises, and apparently thermoregulate by varying convective h eat loss. However, thermal variation in convection has not been direct ly measured for any free-flying bee. Above 33 degrees C, Eying Apis me llifera greatly increase evaporative heat loss with T-a, and many othe r species ''tongue-lash'' during Eight at high T(a)s or when artificia lly heated, Thus, evaporation seems to be important for preventing ove rheating during flight at very high T(a)s. Flying A. mellifera and Cen tris pallida strongly decrease metabolic rate as T-a increases, sugges ting that they are varying metabolic heat production for thermoregulat ion and not aerodynamic requirements. Variation in metabolic heat prod uction appears to be mediated by changes in wingbeat kinematics, since wingbeat frequency decreases with T-a for A. mellifera and Centris sp p, It is unknown if the decrease in Eight metabolic rate at higher T(a )s occurs secondarily as a consequence of greater efficiency or if it is truly an active response.