How do bees keep the hive at the right temperature?

Heating

A bee generates heat by decoupling her flight muscles from her wings and contracting them rapidly — shivering without flying. Individual heater bees press themselves into empty cells among the brood and warm the surrounding pupae directly, raising their own thorax temperature above 40 °C to do it. A pupa a fraction of a degree too cold develops into an adult with measurably impaired learning and dance performance, which is why the tolerance is so tight.

Cooling

• Fanning. Bees line up at the entrance and along the combs, wings beating, driving a directed airflow through the nest. In a strong colony this is audible from outside.
• Evaporative cooling. Foragers switch from nectar to water collection in heat, spread it in thin films across the comb and on cell rims, and fan across it. The physics is a swamp cooler, run by insects.
• Bearding. On very hot evenings a large part of the workforce hangs outside the entrance in a curtain, reducing the number of bodies generating heat inside. It looks like a swarm and is not.

Winter is a different problem

In winter the colony stops heating the whole cavity and heats only itself. The cluster contracts, its density varying with the outside temperature, and the centre is held around 20 °C while broodless — considerably cooler than the summer brood nest, because maintaining 35 °C all winter would consume stores no colony could carry. Once the queen restarts laying in late winter the brood area goes back up to 35 °C, and consumption rises sharply with it.