Bumblebee temperature regulation and honeystomach

Temperature regulation (bumblebees are warm-blooded insects)

Bumblebees often fly in air temperatures as low as 10^oC or lower, yet they cannot take off unless their flight muscles are above 30^oC, and the temperature of the thorax during flight is maintained between 30^o and 40^o C regardless of ambient temperature. So how is this done? Well the hairs do provide some insulation, but to raise their temperature for flight they simply uncouple their wing muscles so that the wings themselves do not move, and use the muscles to shiver and raise their thorax temperature. Then when their thorax is warm enough they can fly.

There is an excellent BBC video showing this. This is why you may find some grounded bumblebees during cold spells in the spring. These are often queens, and you can help them by feeding them or moving them somewhere warm.

Warming up the flight muscles

At rest a bumblebee's body temperature will fall to that of its surroundings. To raise the temperature of the flight muscles high enough to enable flight the bumblebee shivers, rather the same a we do when we are cold. This can easily be seen in a grounded bee as her abdomen will pump to ventilate the flight muscles. The rate of pumping can give an indication of the temperature of the bee. Ranging from around 1 pump per second when she is at 10^oC, to 6 pumps per second when she reaches 35^oC. The time taken to raise the thorax temperature has been studied and is laid out in the table below.

When the queen is brooding in cold weather, probably early on in the nest life, she needs to warm up the bald patch on the underside of her abdomen (brood patch) to keep her eggs warm, so she needs to transfer heat from her thorax to her abdomen. Later the workers will also help to regulate the nest temperature keeping it at around 30^oC. Now all bees, ants and wasps have a very narrow waist (petiole), this isn't very easy to see in bumblebees as their hair makes them look so round and fat, but believe me they have a waist about as thin as a wasp's as you can see in the drawing above right. The heat is transported to the abdomen by the heart pumping warm blood from the thorax through this narrow petiole (waist).

This is also the method used to get rid of excess heat from the thorax on hot days to stop the bumblebee overheating. The heat is dissipated to the outside by inflating and deflating the large air sacs (see drawing above) in the abdomen. The air escapes through tiny holes, called spiracles, that are found down the side of most insects, usually one pair to each segment.

So temperature regulation in a bumblebee body works in a similar way to our bodies when we are cold, i.e. we both shiver to get warm. So it is really not correct to call a bumblebee a cold-blooded animal.