> [the tracheal opening on a honey bee]
> is still somewhat elastic and mites can therefor
> push things around to get in.

This seems a bit of an understatement, given the difference
in size between the typical tracheal mite and the tracheal
spiracles.

When one looks at the calibrated images here:
http://www.sel.barc.usda.gov/acari/content/trachealmites.html

and compares the sizes of the mites with the excellent
micrographs in chapter 6 ("Respiration")  in the late
Lesley Goodman's "Form and Function in the Honey Bee",
one can see that even a 10% reduction in the size of
the spiracles themselves would not seem to prevent a
tracheal mite from entering.

When one then adds in the following clue:

> John McMullen...This study showed that for a body size
> shift of 10% there was only a 1% change in the diameter of
> the tracheal orifice.

one is forced to question if it would even be possible to
"downsize" a bee to get a small enough spiracle to naturally
exclude tracheal mites.

It seems to be a moot point, regardless. Given that multiple
breeders have effectively eliminated tracheal mite susceptibility
in "non-downsized" bees, it seems clear that smaller spiracles
are not required to achieve "tracheal-mite resistant" bees.


        jim

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