"The worm has turned." I know what it means, but what is its origin?

Question

"The worm has turned." I know what it means, but what is its origin?

Answer 1

The meaning was that even the most humble creature tries to counteract rough treatment.

Shakespeare picked up the thought in Henry VI, Part 3, where Lord Clifford urges the king against 'lenity and harmful pity, saying:

To whom do lions cast their gentle looks?

Not to the beast that would usurp their den.

The smallest worm will turn being trodden on,

And doves will peck in safeguard of their brood.'"