Close-up thermographic recordings of bees inside three empty cells adjacent to brood cells. A and B show a view of the same cells at different times. x, y and z each indicate the bottom of one of the observation cells. The white lines indicate the middle wall. (A) Three heating bees with different thorax temperatures (T_th); all of them are seen from their dorsal sides. (B) A resting bee in cell y and a heating bee in cell z (turned upside-down). Cell x is empty. Abbreviations: abd, abdomen; hd, head; th, thorax; w, wing. Asterisks mark the wax walls of two sealed pupae cells adjacent to the observation cells. The temperatures refer to the bee thorax.

Simulation of worker heating with an artificially heated thorax (setup C). The cross-section shows one side of a comb. In different cycles, the thorax was placed inside an empty cell (variation A) or on the comb surface either touching the cap (variation B: d=0 mm) or at a distance of 1.0–1.5 mm from the brood cap (variation C). The locations of thermistors for measurement of thorax temperature (T_th), cap temperature (T_cap) and brood temperature (T_brood) are indicated. Other than shown, the T_cap thermistors were not introduced from brood cell #2 but from adjacent cells in the background that were not used for temperature measurements. Abbreviations: d, distance; m, middle wall of the comb; R, resistor inside an isolated bee thorax.

Thermogram of worker bees in the brood area, as seen in a common observation hive. The sealed area appears grey with no further details in this image; open cells are identifiable by the hexagonal structure of the cell rims. One bee with heated thorax [A; T_th(entry)=37.9°C] is about to enter an open cell adjacent to three sealed brood cells and pokes its head and the anterior part of its thorax into the cell (the dark structure on the heated thorax is the scutellum, seen from posterior-dorsal). Worker bee B has just left the open cell in the centre of the image [T_th(exit)=37.3°C]. The view on cell visitors C and D is partly hidden by bees on the comb surface. The onset of heat production during cell visits is roughly indicated when the cell interior and thorax (visible as a ring-like structure around the dark silhouette of the cool abdomen) start `glowing' with increasing intensity. Note the different intensities caused by cell visitors E and F.

Thorax temperatures (T_th) of three different honeybees in the sealed brood area on the comb surface. (A) A 2 min warm-up (+4.7°C) prior to a cell visit provides a T_th(entry) of 39.4°C. (B) Heat production during a cell visit is revealed by a net warming, with (ΔT_th)_net=+4.1°C. Heating was not resumed during a subsequent short-duration visit (0.7 min) to the same cell. (C) Repeated warmings prior to consecutive cell visits. Asterisks mark periods of 30 s or longer during which the bee's thorax was hidden by another bee and no temperature measurements could be made.

Thorax temperatures (T_th) of different honeybees during long visits to empty cells adjacent to sealed brood cells. Bees that are lying still inside cells may be engaged in heat production (A–C) or resting (D). Heating bees with net temperature changes (ΔT_th)_net=+3.9°C (A) and– 0.3°C (B). During a longer cell visit (C), repeated cooling and heating and short interruptions (arrows) occur. Symbols: ↓, bee enters cell; ↑, bee leaves cell. T_th of bee outside cell (open circles) and inside cell (filled circles) are plotted.

Diagram of respiratory pumping movements of the abdomen of bees inside cells. Pumping movements are drawn as spikes from 0 to 1 at the time of occurrence. (A) Resting bee with T_th=33.4±0.3°C. Thick lines are bursts of several pumping movements, which are shown at a higher temporal resolution in B. (C) Heating bee with T_th=36.4±0.3°C. Note that the time-scale of A is different from that of B and C.