William Golding uses the book's title- Lord of the Flies- symbolically and gives the reader an immediate unpleasant sensation at the thought of flies. Initially it seems a strange title for a novel about a group of schoolboys who are stranded on an island without any adult supervision and adds that sense of apprehension as the story proceeds. There are various symbols including the "pig's head on a stick" (ch 8) as Simon recognizes it to be and the symbols collectively allow the boys to represent a microcosm of society as they struggle with their realization that they do need to do the right and the best thing, but as children they also have a childish need to have fun and act impulsively.
The conch shell represents the closest thing to democracy and is used to call the boys to order, have meetings, make decisions and hear anyone who needs to speak. It gives Ralph confidence as leader and as its power diminishes so too does Ralph's confidence in himself as leader. Piggy's glasses add to the boys' perception of good decision making as they are used to start the fire and, when they are misused by Jack, not only Piggy but the whole group is affected. The signal fire is the boys' hope for rescue and a reason to keep trying to survive because as Jack ironically states in chapter 2, "we're not savages."
The Lord of the Flies itself then is a sharp contrast to all the boys' best efforts and defies everything childlike, innocent and good. Literally, it is the head of the pig which Jack killed so that they can have a feast and invite everyone to it. Symbolically for Jack, it proves his ability to embrace his compulsive side and gain some measure of control. It effectively gives him permission to behave erratically, irresponsibly and brutally. On Jack's instructions the boys sharpen a stick and he impales the head on the stick and leaves the head as a gift for the personified beast which the boys are convinced that they have seen.
For Simon, the Lord of the Flies is the image which he encounters and discredits. Simon is not easily fooled and the pig's head is threatened by Simon's attempts to warn the others about the beast. It reminds Simon that the island is for fun and that he cannot stop the fun. It is clear that the Lord of the Flies version of "fun" is not at all childlike and is violent, vindictive and merciless.