Dear Raül,

Try methylene blue in alcoholic solution.

Thank you very much Manuel. I hope to show in brief

Thank you very much Landeira.

Some silver stains (1% silver nitrte) have been used as well as they can provide contrast (e.g., http://www.nrcresearchpress.com/doi/pdf/10.1139/z85-105), but I've not used them. Reference to their use as exterior stains can be hard to find. You could also try epifluorescence as the cuticle should have a lot of it. I've not tried methylene blue on crustaceans. That's an easy dye to obtain and may provide good contrast.