Answer:
'The adult snake is dead' he's referring to Banquo.
"But the worm that's escaped will mature and become venomous in time--he has no teeth at the moment he's referring to fleance.
Explanation:
Banquo is dead but fleance escaped, Macbeth is saying right now fleance is harmless but once he's older he'll seek out revenge on Macbeth for killing banquo.
