by Andreas Herrmann
Acetic acid works quite well, but where the proteins of the muscles have become hard due to denaturation, acetic acid will not relax specimens. In such cases, a chemical is needed which will break down the proteins. In Germany we use simulated gastric juice for this aim: 1% pepsin, 1% hydrochloric acid and 98% water. The solution (as well as the pepsin powder itself) should be stored at temperatures below 15 °C when not in use.
I use the same glass tubes (10 or 20 ml) as for killing specimens. Put a few beetles into a tube and fill it with pepsin solution. Then store in a warm place such as an airing cupboard or above a radiator (maybe at 30 or 40 °C) and leave for a few days. If the beetles are still stiff after a few days, they can be left for up to a few weeks but I rarely have to do this. More time is needed for larger beetles, or at lower temperatures. Discard the pepsin solution afterwards: it can only be used once.