Countercurrent Heat Exchanger

Honeybees fly at ambient temperatures up to forty-five degrees Celsius when other 'bee-sized' insects cannot. Bees employ an efficient counter-current heat exchanger that apposes two oppositely directed flows of hemolymph in close proximity to control heat loss and heat gain to maintain heat balance. If one flow is of a higher temperature, heat passively flows downhill from higher to lower temperature across the wall separating the flows.

At warm-up, hemolymph flowing aft conveying heat from thoracic 'motors' flows close to hemolymph passing forward in the dorsal vessel, so heat cycles back to the thorax. Retained heat in the thorax helps thoracic muscles warm-up on cold mornings. Honeybees enhance heat exchange by having the aorta make tight spirals in passing through the narrow petiole thus increasing the area of the aorta contacting the backwards flow of hemolymph.

Now for increasing heat loss. Large naked carpenter bees can lose heat readily. Cuticles of head, thorax and abdomen are smooth and devoid of hairs. Smooth surfaces radiate heat. A radiator increases the surface area to transmit more heat to the environment by convection. A bee's body, filled with hemolymph has a high heat capacity. The open pump circulation spreads heat against the inner surfaces of thin cuticles. When carpenter bees fly at about twelve meters per second, the cooling rate of the head may be ten times greater than that of the thorax. A cooler head draws heat from the flight motors in the thorax as the dorsal vessel passes the hot hemolymph forward. Backward coursing hemolymph through the thorax transfers heat to the abdomen, where the thin walls radiate excess heat away into the air stream. In warmed temperature-controlled spaces, carpenter bees increase their flight speeds. When ambient temperatures are lower, they hover close to the ground conserving heat.

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