palordrolap, 1 year ago

Technically the insect in the original gag was a moth, not a fly.

Certain kinds of moth caterpillars eat cloth. Banknotes at the time and location of the original gag were made of cotton fibre paper, (and indeed some places still do this, or did so very recently) so were theoretically as delicious to those caterpillars as cotton clothes would be.

For clothing, mothballs can be used to deter them from laying eggs wherever the clothes are, but it's kind of hard to cram a mothball into a wallet. Also, the money probably already had the eggs on it, which is too late for a mothball anyway.

Thus, if a moth flies out of your wallet, it means that the paper money is long gone because that moth had time to get all the way from egg, through note-munching caterpillar to moth before you opened your wallet.