Hi Faiz! It depends on your objective. Generally, we use KAA, a mixture of Kerosene and Acetic Acid to fix the muscle tissues of general larvae such as wrigglers before placing them  on 80% ethyl alcohol. Placing them directly would blacken and soften the tissues making them hard to handle.

Originally, we add Dioxane (KAAD) but was later removed due to its carcinogenic properties. Attached is a reference from the USDA museum, see pp. 25. I hope this helps, good luck!

Dear Faiz,

If you are going to use the samples for PCR in future, the best preserving  medium is 95% ethanol. That preserves DNA for a long time. Also it is common to use lactophenol in Iran yet, however the phenol of it is harmful. This medium decreases the darkness and makes the specimen suitable for providing microscope slide without using KOH.

As for any insect, to assure a quick penetration of the conservation medium into the different tissues for obtaining an optimal conservation (or fixation), I recommend you either injecting the solution into the body of living individuals or cutting the tegument.

Well all of the experts have given reply with their views, however, if the objective is to determine a desired compound accumulated inside the insect body, first question is: is that compound a regular metabolite(e.g fat, carbohydrate or a specific protein) or is it a xenobiotic (heavy metal or pesticide),

Whatever would be the case the best way in such requirement is to get freeze (preferable in liquid nitrogen or in the most cases freezing just below zero centigrade). You can keep as such for a long time before getting into analysis. Our experience is with pesticides in the insect/ earth worm body.