Bees are one of the few insects that can generate large amounts of heat with their bodies.

An excerpt from the article 19 facts about honey

Hive bees constantly regulate the temperature of the hive by heating it with their bodies or cooling it down by evaporating water to maintain a constant temperature of around 35 degrees Celsius in the honey room. This process continues when the bees are constantly flapping their wings creating a constant air circulation and evaporating the water from the honey to a content of about 18 percent. This is when the sugar concentration in the honey rises above the saturation point, preventing fermentation. The bees then cover the cells of the comb with wax to seal them (honey is hygroscopic).