Hey Enrico,

maybe not the answer to your question but also a promising wing character to check:

http://www.pnas.org/content/108/2/668.abstract

I don’t know if anybody checked the WIP in beetles?

Greetings from Stuttgart

I really don't know about colouring methods, because i never used. Probably "negro de clorazol" will be useful, because it used for the chitin colouring. But all wing lamina is chitin... for mount i used only water for make them soft, in case if you mount from the dry material, from the alcohol its also possible, but from the 96 - 100% is much more difficult, due to material is very fragile. 70 - 50% is better. I used one thin pincer, for body fixing, and with the another was open elitra and took the wing. In this moment mostly important is not to rip the basal part of the wing. When you hold wing in the pincer, put it on the glass and unfold carefully - in Limnebius apical part is very thin and mostly dangerous, but how is about the Mordellids i don't know.  Finely i find that cover wings with DMHF is the best way to preserve them.

Hi enrico

Try to mount the wings using the methods of Wirth and Marston (1968) "A method for mounting small insects on microscopic slides in Canada Balsam". I mounted Paederus beetles (Staphylinidae) and Culicoides (Ceratopogonidae) wings and I had no problem. Greetings.

Try to mount beetles wing using  liquid paraffin as a mounting media   

I think it will depend on the characters you want to study. Canada Balsam or glycerol would be ok for studying the venation, but sometimes folding lines are also relevant in beetles and you cannot see them if the wings are embedded. For this purpose it is much better to open the wings on a slide in water / alcohol and let them dry. Then you can examine the wing with oblique lighting from above.

I agree with Andrey Rudoy, DMHF is the best way to preserve but study Diptera and, as Alexander Riedel says, also I give importance to the folds (not only the veins), and to observe them is very useful light from the side, following the same technique that says Alexander. So depending on family / genus I use one or the other (I do not always interested in preserving the sample permanently).

Thoroughly relax your specimen in distilled water, with a little alcohol or detergent to ensure wetting. Spread the elytra and dissect off the wing, making sure you get the basal sclerites. Place the folded wing in a drop of distilled water on a glass slide, and carefully spread it open. Remove the water with the edge of a piece of absorbent paper. Check that it's really flat, then air dry for up to 2 days. For a permanent prep, flood the dried specimen with mounting medium and cover with a cover glass. If the veins are thick you'll need to support the cover glass with glass filaments.  Sometimes unfolding the wing is very difficult on a glass slide. In this case do the first spreading on  a piece of flat smooth plastic or card and secure it with minuten needles inserted alongside the major veins. After it's dried you can remove the needles and transfer the flattened wing to a slide, if necessary adding some water to relax it again for final adjustments.  It's a great help to always mount the wing with the dorsal surface uppermost - saves later confusion. 

Further to my last answer: sorry, I missed the query about staining. As Andrey wrote, chlorazol black would probably work, but for most chitin membranes and sclerites I prefer acid fuchsin in 5% acetic acid. Leave it till darkly coloured, then differentiate in tap water or mild alkali to the desired degree. 

Hugh Rowell

The best method, to me, is the above proposed  by Alexander Riedel. In the carabid beetles I study, fortunately, the anterior wings (elytra) have characteristics visible with naked eye ant the fliyng wings are all alike, so I never had the need to study them by other means. .

Hi, Enrico, Regarding the mounting methods see Fedorenko D.N. 2009. Evolution of the beetle hind wing, with special reference to folding (Insecta, Coleoptera). Pensoft, Sofia-Moscow. 336 pp. I use water for temporary slides and then glue the wing on the cardboard to keep it together with specimen (for valuable specimens). For common species I use permanent slides with any mounting media and then study under the transmission optic microscope. It's good for wing venation, basal sclerites, secondary sclerotization, microsculpture, and even folds.

An old and effective technique for larger wings is to mount them dry between two 2x2 inch glass slides, sealed at the edges with narrow aluminum tape.  They can be projected like an old transparency and easily studied and drawn.

A crude way of looking at cross-sectional detail is to embed them in clear casting resin and then chop them up with into slices and polish the end faces. This allows you to examine a TS at a particular location, but also to be able to pan down over the lower adjacent area and see the detail sideways on.

Can you look at them on a Scanning Electron Microscope?"

Soaking in Teepol detergent for an extended time will remove pigmentation leaving  a transparent cuticle (well it did in dragonflies!.

Sorry if this duplicates other answers - no time to review them,  Best wishes David