As a kid I used to go insect collecting with my father. He would have a number of glass tubes, at the bottom of the tubes were what he always decribed as 'laurel leaves', crushed up and held down with cotton wool. Any insect in the tube would die within seconds if the leaves were fresh.
I suppose they could have been leaves of something else, but thinking back at the shape of them, I suspect they might have been bay leaves. Not tested this theory out though
I imagine this may have been a fairly common practice for collecting insect specimens some years ago.