> >It seems hard to imagine that all of this could be retained indefinitely
> inside the cluster.
>

No need to try to imagine : )
I've written about this subject in the last few issues of ABJ, and will
soon post the articles to my website.
Some snips:

The “winter bees” in the cluster have little need for protein, due to their
well-developed fat bodies, and can survive for a long time on a diet of
honey (sugar) alone. The weight loss of a hive in winter cluster (not
rearing brood) is in the ballpark of around a pound a week [at 5C, more if
colder or warmer]― presumably mainly due to the consumption of its honey.
That honey consists of roughly 83% sugars (mostly glucose and fructose) and
17% water. The bees metabolize only the sugars, according to the following
equation:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

When you do the math, the metabolism of the sugar in that pound of honey
produces 6/10 of a pound of water. Add to that the 17% of liquid water
already present in the honey, and you wind up with that pound of honey
turning into 2/3 of a pound (1¼ cups) of water (initially held within the
bees’ bodies). The bees in the cluster cannot allow that 1¼ cup of water to
accumulate, and must deal with it in some manner.

Bees in the winter cluster have only a few options for what to do with the
water in their bodies resulting from the consumption of honey; they can
either:
1.       Hold it in their hind gut (up to a point), or
2.       Defecate it (not a desirable option in the cluster), or
3.       Exhale it through their spiracles via “respiratory transpiration,”
or
4.       Feed it out through their proboscis (generally to another bee).

In the final article I describe Omholt's explanation (currently pretty well
supported by the observations of others) that the bees in the cluster,
rather than depending upon passive diffusion of moisture, instead actively
control the forced ventilation of moisture-laden air out of the BOTTOM of
the cluster, rather than the top.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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