The Fly as a Life Index.
The fly here represents the spirit, an idea very common in folk-lore, where an insect often appears as the Life Index. An English lady has been known in India to stop playing lawn-tennis because a butterfly settled in the court. In Cornwall wandering spirits take the form of moths, ants, and weasels.[195] We have the same idea in Titus Andronicus, when Marcus, having been rebuked for killing a fly, gives as his reason,—
“It was a black, ill-favoured fly,
Like to the empress Moor; therefore I kill’d him.”
A fly is the guardian spirit of St. Michael’s well in Banff.[196]