The conch becomes the symbol of authority and law and order. It's used to call assemblies and as a kind of microphone that grants the right to speak to whomever holds it during assembly.
By allowing the boys to create fire, the first necessity of civilization, Piggy's glasses represent science and technology, mankind's power to transform and remake their environment to best suit its needs.
Like the glasses that create it, fire represents technology, it is also a technology that threatens destruction if it gets out of control. Fire also symbolizes the boy's connection to human civilization: their signal fire gives them hope of rescue.
This is the pig's head that Jack impales on a wooden stake in sacrifice to the beast.
"Lord of the Flies" is a literal translation from Hebrew and means Beelzebub.
The Lord of the Flies is both a physical representation of "the beast", as well as a symbol for Satan himself.
The plane crash symbolizes the breakdown of modern society. The boys leave England to get away from the war and are shot down by the enemy. The scar left by the plane crash, symbolizes the damaging encroachment of humans in paradise.
The boys also stand as symbols: Simon represents goodness; Ralph and Piggy symbolize law and order; Jack and Roger stand for evil; The big kids represent the ruling classes; and the littluns symbolize common folk.