> And how do you define pain and pleasure?

They're not reducible, but I don't know if that means we don't have definitions; we can describe them well enough that most people (who aren't p-zombies or playing the sceptical philosopher role) know pretty well what we mean. All of our definitions have to bottom out somewhere...

> Do insects feel pain?

Nobody (except the insects) can know for sure. Our inability to know whether X is true doesn't imply X is meaningless, though.