A Coster (also called a Costermonger) is an old English name for someone who sells fruit and vegetables etc. from a barrow. An odd name to use!

I suspect that is used to be Coaster (not because it is found by coasts, but because of its flight which is usually weak and can be described as coasting along), but, like the Sailers/Sailors, it got corrupted over time and the 'wrong' version got into general usage.

Just to confuse matters further, is A. terpsicore the same species as mentioned in D. A. Swanpoel's Butterflies of South Africa (1953) as Acraea terpsichore Linnaeus 1758, which, in Funet, is considered a synonym of A. serena? Funet mentions ssp. of A. terpsicore (note change of spelling for the ssp.) named neobule, seis, arabica and legrandi, but includes them as ssp. of A. neobule. A further ssp. A. terpsicore mahela is also mentioned as a seperate species, A. mahela.