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i've used cotton plugs for this. Shove the cotton into the tube, down 
into the fixative - the cotton wicks up fix and physically holds the 
sample submerged in the fixative.
But I also suspect air in the trachea is what's keeping the tissue up 
(if there is no exoskeleton on the samples, then I'd suspect waxy 
lipids on the cuticle), so pulling a gentle vacuum should work. You 
might have to use some vacuum/vent/vacuum/vent cycles.

Phil

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>Hi all,
>    Bit of an off topic question, but we do lots of immunos on small bits of
>Drosophila tissue that always float on top of the fix due to the surface
>tension.  We can use low amounts of triton (.01%) to overcome this, but I
>would like to be able to leave detergents out until I am convinced the
>fixation is good.  In some cases I would like to do staining of only things
>on the cell surface, and thus need to leave detergents completely out of the
>staining procedure.  Therefore, does anyone know of a good surfactant that
>would break the surface tension but not permeabilize cells?
>
>Thanks,
>    Sean Speese

-- 
Philip Oshel
Microscopy Facility Supervisor
Biology Department
024C Brooks Hall
Central Michigan University
Mt. Pleasant, MI 48859
(989) 774-3576