During what seems likely to have been an epileptic fit Simon held an internalised conversation with the evil or darkness within himself, while staring at the pig's head mounted on a stick.
Jack leaves the guts of the sow and its head, mounted on a stick, as an offering for the beast.
if you mean the beast then it is a pigs head on a stick
Lord Of The Flies, it is the head of the pig offered as a sacrifice to the beast.
When Jack and his tribe kill the pig they stick the pigs head on a spear and put it in the clearing as an offering to "the beast". They do not know that the beast is actually a part of them and not a physical being.
The lord of the flies is the head of the pig that Jack and his 'tribe' killed, they left it on a stake as an offering to the beast (ie)
Having just looked at the book I can't find repeatedallusions to the buzzing of the flies but I think that their buzzing could perhaps echo the pulse which began to beat on Simon's brain, which is referred to in the the passage where Simon has his epileptic fit and converses with his internal 'Lord of the Flies.' So I guess you could say that the reference to the buzzing of the flies echoes the buzzing in Simon's own head, just as the illusion of the Beast talking to him from the head of a dead pig is an echo of his own internalized conversation.
It symbolizes the monstrosity and animalism of everyone on the island, when it was meant to be an offering to the beast. The beast was in all of them.