Killing jar

One design has a thin layer of hardened plaster of Paris on the bottom to absorb the killing agent.

The absorbent plaster of Paris layer also helps prevent the agent sticking to and damaging insects.

A piece of stiff paper or cardboard cut to fit the inside of the jar tightly is then pressed in.

If the jar is not used for long periods it may dry out and produce little gas; a few drops of water will help get the process going again.

In former times, amateur entomologists commonly used the thick green leaves of the Cherry Laurel (Prunus laurocerasus or Prunus caroliniana) which, crushed or finely sliced, will similarly release hydrogen cyanide.

[5] Butterflies and moths are generally killed manually by crushing the thorax, as they can often destroy their wings by beating them against the jar.