Lifesaver or Deadly Poison?

Violet Oil Beetles Meloe violaceus will be out in the ancient woodlands soon, seeking each other out to mate. These lovely irridescent beetles are very visible for a few weeks, often around Wood Anemone Anemone nemorosa, which is flowers at this time of year, and is their favourite food.

Male oil beetles have 'elbowed' antennae, used for gripping the female.

Oil Beetles are so called because they are part of a family which protect themselves by synthesising a deadly poison called cantharidin in their bodies. If threatened they ooze orange 'blood' from their joints. A minute amount ingested by a mammal works like viagra (Spanish Fly is in the same family of beetles), but even very small doses can kill, as it is as poisonous as strychnine. The cantharidin laden body fluid most commonly causes blisters (hence their other name of Blister Beetles) when the beetles are touched. Mostly though, predators know to avoid these dangerous creatures.

Mating Violet Oil Beetles in the Forêt de Preuilly, the much larger female on the left.

Female Oil Beetles don't initially have much cantharidin. They receive it as part of the sperm package when they mate, and then coat their eggs with it to protect them. Male beetles are packed with the poison, but the exact process by which they manufacture the cantharidin is not fully understood.

Recently there has been some promising new research, and it seems that cantharidin may be a useful anti-cancer drug. It's early days yet in the research, but mice with cancer have been treated with a compound extracted from meloid beetles, and the results have been remarkably good.