Sweet are the uses of adversity:
Which, like the toad, ugly and venomous,
Wears yet a precious jewel in his head. [ William Shakespeare ]

An asp would render its sting more venomous by dipping it into the heart of a coquette. [ Poincelot ]

Some read books only with a view to find fault, while others read only, to be taught; the former are like venomous spiders, extracting a poisonous quality, where the latter, like the bees, sip out a sweet and profitable juice. [ L'Estrange ]