Thermoregulation of swarm beeballs

After eating plenty of honey, bees fly out from the nest and make a swarm ball on a thick branch or the like. They wait still until exploring bees find the place to make their new nest. It seems better that they keep the temperature of the swarm beeball as the same degree as the air temperature to save the energy.
However, take Western honeybees for example, they always keep the temperature of inside of swarm beeballs around 36℃. There is a difference in shape between Western honeybees' beeballs and Japanese honeybees'. And Japanese honeybees' beeballs look more compact.
Then, I measured the temperature of Japanese honeybees' swarm beeballs. Using an ordinary thermometer may be a risk that bees fly away, so I used a thermocouple thermometer (digital pocket type H-03). As shown in the table below, the temperature of the core of the Japanese honeybees' beeballs were also around 36℃ as well as Western honeybees'. It is thought that they have to keep the temperature for baby bees immediately after hatching, which live in the brood nest of comb, and for the queen.

Temperature measurement of a swarm and a abscond beeball

i	Days	Ambient Temp.	Core Temp.	Time	Note
Swarm Beeball	1999.4.20	23.4	35.6℃	16:00	Attracted to aPrunus Mume at Hirakata
Swarm Beeball	1999.4.25	19.4℃	34.9℃	14:32	Attracted to a Cedrus Deodara at Moriguchi
Swarm Beeball	1999.4.26	16.9℃	35.2℃	19:34	Attracted to a luring board at Moriguchi
Swarm Beeball	1999.5.14	27.7℃	34.4℃	17:25	Attracted to a Pinus Thunbergii at Moriguchi
Swarm Beeball	1999.6.27	23.8℃	35.6℃	15:35	Attracted to a Machilus japonica at Moriguchi

Structure of the swarm beeball

Natural swarm beeballs can be seen at various places and heights. Beeballs on a straight thick trunk cover the surface of the tree like cloth. On the other hand, beeballs on a horizontal thin branch dangle like a piece of board. It is impossible to observe the structure of swarm beeballs with these natural ones.

a swarm on a trunk standing straight	a swarm hanging down from a horizontal branch

Then, I made a piece of equipment named by myself -the swarm luring board. It is a 45cm square piece of board covered with a stainless steel net with fine mesh. On the surface, bee wax is attached. The swarm luring board has strong possibility of attracting a swarm. Besides, almost every swarm on this equipment is shaped into excellent hemisphere and has an excellent structure. Although natural swarm beeballs are certainly the same in structure, no one has been noticed because of their various shapes. The surface of a swarm beeball is covered with single-layered honeybees linking legs each other. When other honeybees come home, they pass through between the surface bees and enter the swarm. There is a difference in body temperature between surface bees and inside bees, and surface bees' temperature is always lower than inside bees'. When the night comes and the temperature falls, surface bees put their heads into the bottoms of the front bees. They make the swarm beeballs compact by closing up chinks between them in order not to lose the inside heat. 　

a loose beeball ( daytime)	a compact beeball(nighttime)

Dance on a swarm beeball

The Western exploring honeybees tell their family members a suitable spot for their new nest by dancing on the bottom of the swarm beeball. However, I rarely see Japanese honeybees' dance. Because, in case of the hemisphere beeballs made on a swarm luring board, exploring bees usually get into the beeball and dance behind the surface-layered bees when they come home. Exceptionally, you can see the dance on the surface of a beeball when the surface structure gets partly out of order. It is hard to observe the Japanese honeybees' dance.

a dance can be seen occasionally.	dancing bees

The temperature change of swarm beeballs until takeoff

When the dancing bees' information spread among the family, it is time to leave for the new place to live. However, they do not get off immediately because they have to raise their body temperature. I had a chance to observe that a small swarm, which had formed on May 3, 2000, flew away on May 4. Early in the morning, the surface temperature of the small s
warm beeball was still 15℃. When the morning sunshine came, the exploring bees began to work and the surface temperature of the ball gradually rose up. An hour before the departure, honeybees were moving around on the swarm, stimulating each other. When most honeybees' temperatures rose to 41℃, they took off in a body. (Enlarged thermo grams are here; photo1, photo2)

Am 5:15 Tep. 9.3 Am 7:06 Tep.11.7 Am 8:06 Tep.13.4 Am 9:06 Tep.15.9

Am 10:00 Tep.18.5 Am 10:15 Tep.20 Am 10:30 Tep.19 Am 11:00 Tep.19.5

In May 2001, using four thermocouples and a multichannel recorder in addition to a themography, I measured the temperature of three parts of a swarm beeball and the air temperature simultaneously. A swarm beeball on a KINRYOUHEN and three beeballs on swarm luring boards were measured. Some of the result graphs were shown below. The temperature of the surface honeybees are dropped as the swarm beeballs are formed. When the air temperature fall, the inside of the swarm beeballs are kept at 36℃. Next day, as the air temperature rise and bees begin to work, the body temperature of bees gradually rise and reach 41℃ at the point of just before taking off. The same are the case with the swarms on a KINRYOUHEN and swarm luring boards.
In 1981, B.Heinrich measured the temperature of Western honeybees during the preparations for the trip by the same way. According to his record, the temperature of all the Western honeybees of the swarm was 36℃ at the point of taking off. It is 5℃ lower than Japanese honeybees'. In Both Western and Japanese honeybees, the temperature of the inside of the swarm beeballs are 36℃, however, there is a big difference in the body temperatures at the point of takeoff . I am very interested in what cause of this difference.