I dissected thirty bees this afternoon - ten from my hive that is having problems, ten from my strongest hive, and ten from a recent cut-out. It took me a while to get the process worked out, but once you figure out how to remove the head and cervical structure properly, getting at the tracheal tubes is pretty easy.
The results were dramatic. Practically nothing for my strongest hive (in fact, it may really have been nothing, but there were some suspicious dark patches, which might represent scarring or dead mites), practically nothing from the cut-out (two bees had what I think is minor mite infestation), and terribly clogged, scarred, darkened tissue in all of the bees from the sick hive. I would also swear there is fungus in there, but I will have to confirm that by preparing a slide for higher magnification tomorrow.
So, I have a bad case of tracheal mites and I guess I should treat them with a miticide. The question is: do I treat all my hives, or just the sick one? Should I expect this to spread, or is this an opportunistic infestation of a hive that is genetically vulnerable or already weakened by another factor?