When you see butterflies or insects pinned down underneath glass, is there something that is done to ensure they won’t decompose? Can they last without decomposing for a limited amount of time or will an insect pinned under glass last for decades or longer?
As long as it’s kept dry, a pinned insect will keep indefinitely. However, if they are in a humid atmosphere, they can mold up and ultimately fall apart. Also, if dermestid beetles get at them, they will chew them up.
My mother still has my old insect collection in her basement, with no climatic safeguards whatsoever. After about 45 years, most of the insects are still in pretty good shape, especially the butterfly wings. Several of the bodies have decomposed, but not as many as you’d think.