Bumblebees often fly in air temperatures as low as 10oC or lower, yet they cannot take off unless their flight muscles are above 30oC, and the temperature of the thorax during flight is maintained between 30o and 40o 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.
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 10oC, to 6 pumps per second when she reaches 35oC. The time taken to raise the thorax temperature has been studied and is laid out in the table below.
|Bee/air temp. oC||Time taken to reach 30oC|
|24||a few seconds|
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 30oC. 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.
Bumblebees gather nectar into their honeystomachs to transport it back to the nest. The honeystomach (see above) is located in the abdomen, and it is just a cuticle-lined bag with a long neck located at the mouthparts. It holds 0.06 - 0.20 ml, depending on the size of bumblebee, and when full can take up as much as 95% of the abdominal space and hold 90% of the body weight.
During foraging the bee needs energy, so she will consume some of the contents of the honeystomach. To allow her to do this there is a small valve at the end which can allow some of the nectar to pass into the bee's own digestive system. It has been estimated that a full honeystomach will give a bumblebee about 40 minutes of flying time.
Some flowers contain as little as 0.001 ml of nectar, so to fill her honeystomach the bumblebee may have to suck nectar from 60 flowers, and to find these 60 she may have to visit 100 or more. Then she will return to the nest, which may be as much as two miles away. So providing a supply of nectar for her nestmates would not be possible without the honeystomach to carry it in. A teaspoon holds about 5 ml and nectar is about half water, so to fill a teaspoon of honey a small bumblebee might need to make over 80 foraging trips, flying up to 320 miles, and sucking 80,000 flowers! Honeybees also have a honeystomach, and as they are smaller than bumblebees they would have to make even more foraging trips. Think of that next time you spread honey on your toast!