I have an observation beehive. I can open up a window and watch the bees on the brood comb at my leisure. At any time during the day lately there will be numerous bees doing the ‘waggle dance’. If you watch closely some bees are doing the same dance others their own. The thing I realised the other day as I closed the window after some viewing was: ‘as soon as I close this window it’s going to be pitch black dark in there’. Yet I have zero doubt that the waggle dancing continues unabated.
What then dawned on me was that bees do not watch another bee waggling - somehow they sense it. Vibrations seems the most obvious mode. Considering the complex information that’s encoded into the dance- the system for deciphering the vibrations must also be highly tuned. I wouldn’t discount for one second Dawn’s explanation as THE explanation you are looking for. When they walk over the cells the difference between hollow ones and full ones must be instantly ‘obvious’ to a bee.
You could also ask: ‘how is it that bees build such perfect complex geometrical comb- in the pitch blackness of the hive?’.
Bees are used to walking over comb in all states and containing many things: pollen, honey, larvae, capped brood. There entire lives revolve around managing the comb- always sensing what needs to be done with any given cell they come across. It doesn’t surprise me in the least that they instantaneously recognize that their beautiful huge packed frames of honey are suddenly mysteriously empty and hollow feeling. We harvested flow frames a few weeks ago- and you could watch the bees furiously tearing up and shredding the cappings even as the honey flowed out the back of the hive. You can watch the small showers of wax flakes falling down.
A final thing occurred to me- I suppose from time to time capped brood must die before they emerge as bees. The bees must be able to sense when this happens and uncap and clean out the remains. This could be a similar process to what happens with a flow cell- perhaps explaining how they instantly ‘adapted’ to it.