Have butterflies in stomach = feel very nervous.

• I always have butterflies in my stomach just before an exam.
• As it was his first stage performance he had butterflies in his stomach.

The law of the jungle = a situation in which there are no laws or rules to govern the way that people behave.  A state of ruthless competition or self interest.( A system or mode of action in which the strongest survive, presumably as animals.)

• The recent price war among airlines was governed by the law of the jungle.

A can of worms = A situation that causes a lot of problems for you when you start to deal with it.

• Taking that girl home turned out to be a can of worms.
• Corruption is a serious problem , but nobody has yet been willing to open up that can of worms.
• I think if we start asking questions we’ll open up a whole new can of worms.  Perhaps we should just accept the situation.