Symbolism, Imagery, Allegory
Flam. Lo you, sister!
Stay, my lord; I 'll tell you a tale. The crocodile, which lives
in the River Nilus, hath a worm breeds i' th' teeth of 't, which puts
it to extreme anguish: a little bird, no bigger than a wren, is
barber-surgeon to this crocodile; flies into the jaws of 't, picks out
the worm, and brings present remedy. The fish, glad of ease, but
ungrateful to her that did it, that the bird may not talk largely of
her abroad for non-payment, closeth her chaps, intending to swallow
her, and so put her to perpetual silence. But nature, loathing such
ingratitude, hath armed this bird with a quill or prick on the head,
top o' th' which wounds the crocodile i' th' mouth, forceth her open
her bloody prison, and away flies the pretty tooth-picker from her
cruel patient. (4.2)
When Flamineo first says this, Brachiano thinks he's arguing that he hasn't been properly rewarded for aiding the Duke. Flamineo says that's not what he means. He claims that his sister is the crocodile—her fame has been ruined, and the Duke is repairing it by setting her free from imprisonment and marrying her. She should remember to be grateful to the Duke, or else he might sting her in some way: like the quill on the little bird's head.
Basically, he's likening his sister's relationship with the Duke to a symbiotic relationship between animals in nature: the bird gets something to eat, the crocodile gets its pain removed. It's a win-win, an "I-scratch-your-back-you-scratch-mine" kind of deal. This is telling, since it shows how these people relate to each other (or how Flamineo sees people as relating to each other): it's all about what you can get out of the other person. If someone stops giving you what you need, in Flamineo's eyes, you'd be justified in dispensing with them.