> Do insects feel pain?

Yes, I think so. Because they show behavior that is consistent with being in a state of pain.

Despite what consciousness really is, I think evolution found a way to tap into that, by causing pain, or by registering pain on the consciousness by some unknown mechanism, for behaviors that are not beneficial to the organism that hosts the respective consciousness...

So I think if an organism that evolved here can display painful behavior, then it should really feel pain.

> 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.