In Lord of the Flies, what is the joke that the pig's head refers to & what is Simon 'wrong' about when the pig's head says 'You were just wrong,...

This is definitely a tricky passage, because the reader
must decipher between the conversation between Simon and the pig's head occurs
completely within Simon's own thoughts.  When the head silently says "It was a joke
really--why should you bother?," Simon is justifying the other boys' actions and his own
to himself.  He seems upset about the "indignity" of the head on a stick, that the other
boys defiled nature that way, so he excuses their behavior by saying it was just a joke,
that it did not really mean anything. 

Simon considers the
head on a stick, wondering if the beast might come back for it, and he recalls his
original idea-- the concept of the beast being within the boys themselves; this is the
thought that he wonders if he was wrong about.  Later, in his conversation with the Lord
of the Flies, the pig's head will confirm that Simon was correct, not wrong, but all the
jungle seems to laugh mockingly at him, as if the Lord of the Flies realizes that
Simon's epiphany has come too late to save him or any of the other boys.