...is caused by lipids in the body leaching into and staining the white maculations. It's especially a problem with specimens killed in ethyl acetate. You can eliminate this by immersing the specimens in solvent for a couple days to completely remove the lipids. I prefer ethyl acetate because it keeps the specimens relaxed, but others use acetone - if you use the latter you'll need to follow that with another immersion in Barbers fluid to relax the specimens. It's easiest to do all this before pinning (note - acetone will dissolve some pin heads).