I just realized this. Heracross is bug and fighting. Why does psychic hurt him if bug hurts psychich? Ludicolo is water and grass, yet fire doesn't hurt him. Why?
OK let me put this simply: Bug isn't resistant to psychic so the Fighting side of Heracross gets hit by the super effectiveness, but in Ludicolo's case, water is resistant to fire so the grass side of him is not hit as hard.
:D hope this helps.