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Although it was labelled Cantharis, with the word "Russian" (for Russian fly) written below, the iridescent insect turned out to be the beetle known as Spanish fly.

This produces cantharidin, which is secreted by the male beetle during copulation and can cause extensive blistering of human skin.

The substance can be used to remove warts or tattoos effectively, but it also has a more colourful past.

If ingested, cantharidin irritates the urinary tract, causing the genitals to swell. This can lead to priapism in men, resulting in erections that can last for more than four hours.

It was famously used by the Marquis de Sade, who in 1772 offered aniseed sweets laced with Spanish fly in an attempt to exploit mistaken beliefs about its aphrodisiac properties.

Send suggestions for this series on the sector's treasures, oddities and curiosities to: matthew.reisz@tsleducation.com.