As an entomologist with a focus on insect biodiversity, I can tell you that not all beetles are poisonous. However, there are certain species within the order Coleoptera that possess defensive mechanisms which can be harmful to potential predators. One well-known example is the blister beetle, which can secrete a toxin that can cause skin irritation and, in some cases, more severe reactions if ingested. Another example is the bombardier beetle, which can emit a hot chemical spray from its abdomen as a defense mechanism. It's important to note that the vast majority of beetles are harmless to humans.

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Cantharidin is the poisonous substance in blister beetles. It is comparable to cyanide and strychnine in toxicity. Although horses are considered to be very susceptible, comparable doses can poison cattle or sheep. Very small amounts of cantharidin can cause colic in horses.read more >>