American children are used to being told that something gives adults the ‘creeps’. As a child in Britain before the Second World War, I was often told by grown-ups that I ‘gave them the willies’. It’s the same thing. But I was confused because ‘willy’ was the accepted child euphemism for ‘penis’. Thanks to The Dancing Goddesses I now know that ‘willies’ are vili or rusalki: the spirits of girls who died before marriage and, since their fertility had not been discharged (as it were), became a special kind of ancestor spirit, at once powerful and dangerous. In southeast Europe and Russia, where these beliefs were most prevalent, the rusalki were water spirits – beautiful slender girls with long dark hair, diaphanously clad (sometimes with detachable wings), emerging from lakes and streams at crucial times to dance, swing and bestow or withhold fertility on the villagers. They appeared in folk tales and art across Europe as naked girls, sometimes with fishtails, sometimes wings or claws, sometimes all of these. If discovered without their chemises and wings they could be captured and married, but inevitably they retrieved their accoutrements and literally flew away.

The ballets Giselle and Swan Lake are adapted from these tales, and Dvořák wrote a whole opera called Rusalka. Vilja, about whom a ballad is sung in The Merry Widow, is one of them, as is Heine’s Lorelei, and the Greek Naiads and Nereids. Even Ophelia has some obvious connections: