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There's a species of moth that has for hundreds of millions of years eaten nectar, but as soon as humans encroached on their home turf they decided to all become vampires and now suck human and other animal blood. Painfully, as they have no anticoagulants nor painkillers like mosquitoes and vampire bats do, they have to rock back and forth to dig their mouth-tube under your skin until they hit a vein, which they've apparently never done until they met humans. (No evidence they snacked on other animal's blood until after humanity's first contact.)