In 1881 red admirals appeared in Russia in very large numbers, an event that was thought to presage the assassination of Tsar Alexander II that year. The Russians could see the numbers 1881 on the butterfly's underwings. Here is a red admiral upside down on my buddleia. It does appear to have the number 18 on its hind wing and I suppose that would read 1881 from underneath with its wings outstretched.
The red admiral gets its name from a corruption of "red admirable". Its scientific name Vanessa atalanta was chosen by Linnaeus in 1758. The name Vanessa was made up by Jonathan Swift in a poem Cadenus and Vanessa written in 1712 and first published in 1726. You don't meet many admirals called Vanessa!