Wing condition and age.
The condition of the wings can be used to age a bumblebee to some extent. Look at the bumblebee on the left, notice that the edge of her wing is damaged. This is typical of an older bee. It doesn't seem to hinder their ability to work. Many bumblebees I have examined that have had ragged wings have also had bald patch on the thorax (though this is very rare in Bombus pascuorum), and in sunny summers their hairs have faded in colour. I think this is a Bombus humilis worker, but as I didn't capture her I cannot be sure. She was photographed in the centre of Paris, just outside my study window. Her pollen basket is partially full, and she, or her nest mates returned to the pot of flowers regularly.

When a bumblebee can and cannot fly
I'm often asked about the story that bumblebees shouldn't be able to fly but can. Well there is a link on my FAQ page to an full explanation of the story, but the gist of it was that the engineers involved were using figures and equations as applied to fixed-wing aircraft, but the bumblebee wing is not fixed, it can flap and swivel. So it would have been more accurate to use the aerodynamics that are applied to helicopters. However a bumblebee cannot fly if its wing muscle temperature falls below 30^oC. In flight the muscle temperature is regulated to stay between 30 - 44^oC. And all this is powered by the sugars found in the nectar the bees suck from flowers.

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 food is plentiful and outside temperatures fall below 10^oC bumblebees generally stay inside the nest and live off their stores. At times when food is scarce or stores are low they will forage when the outside temperature is as low as 6^oC, and queens will forage at even lower temperatures. In severe conditions they have even been known to vary their flying height to and from the nest to take advantage of any temperature differences.

Weight
Bumblebee workers can weigh from 0.04 g to 0.60 g, and some large queens can reach 0.85g. Bombus lucorum workers range from 0.04 - 0.32 g and the queens from 0.46 - 0.70 g. Some of the early workers of smaller species are really tiny, and no bigger than houseflies. In other species, or nests where food has been plentiful the workers can be almost the same weight as the queen. In addition to body weight, female and queen bumblebees can carry as much as their whole weight again in pollen in their pollen baskets, and nectar/honey in their honeystomachs.

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