“The Loves of the Thunder-Gods.
Two young thunder-gods, sons of the chief thunder-god, fell violently in love with the same Aino woman. Said one of them to p. 8 the other, in a joking way: “I will become a flea, so as to be able to hop into her bosom.” Said the other: “I will become a louse, so as to be able to stay always in her bosom.”
“Are those your wishes?” cried their father, the chief thunder-god. “You shall be taken at your word”; and forthwith the one of them who had said he would become a flea was turned into a flea, while he who said he would become a louse was turned into a louse. Hence all the fleas and lice that exist at the present day.
This accounts for the fact that, whenever there is a thunder storm, fleas jump out of all sorts of places where there were none to be seen before.—(Written down from memory. Told by Ishanashte, 27th November, 1886.)”
Source: Basil Hall Chamberlain’s “Aino Folk-Tales”
Compare the above Ainu folktale with the mainstream Japanese ones, Raijin and Raiju