Previously, potassium cyanide was used as an active ingredient; because of its toxicity this is now usually considered obsolete. The potassium cyanide was put under the plaster of paris and slowly decomposed, giving off hydrogen cyanide fumes. Certain millipedes give off hydrogen cyanide as a defensive mechanism, and placing some species in a jar is sufficient to make a killing jar, although the millipede itself will eventually be killed by the fumes.
Ethyl acetate (a component of nail polish), sprinkled on the bottom of a tightly sealed jar into which some plaster of paris has been poured and has hardened will provide effective results. If the jar is no longer effective it can be replenished with another few drops.
It is important not to have fluid sloshing around in the jar or the insects will get sticky and hairy insects will get matted fur or lose hairs or wing scales (butterflies).