Now that we have had two hard (dahlia-killing) frosts with daytime temperatures hovering in the low 40s all week, most of the insects have disappeared from the backyard. I went out to photograph fall flowers, and found exactly one bee that was brave enough to venture out in the cold sunlight.
Bumblebees can fly at low air temperatures, but only if their thoracic temperature (where the wing muscles are located) is between 30 and 40 C (86-104 F). To raise their thoracic temperature, they uncouple their flight muscles from their wings, performing a sort of an isometric exercise that generates heat but produces no visible action. However, according to information on the bumblee.org website, it would take 15 minutes for a bumblebee to raise its thoracic temperature from 6 C (like it was today) to 30 C.
In addition to internal heat production by flight muscles, it looks to me like bumblebees might have a secondary system of passive heat gain through the abdomen to speed up the warming process. By spreading their abdominal segments widely, a larger surface area of underlying black cuticle between the rows of hairs is exposed to the sun.
For more information on this topic, BBC Nature Wildlife has a set of videos on hot-bodied Bumblebees from their series on Life in the Undergrowth.