The flies probably symbolise the boys. While the insect flies are attracted to and cluster around the pigs head and the metaphorical evil which it represents the symbolic flies (the boys) cluster around Jack and the real evil which he represents.
jack
In the end of lord of the flies piggys glasses broke into peices.
piggys glasses, so it can start a fire.
piggys glasses and dry wood
they use piggys "specs" to start the fire (sunlight unto glass). This is why in the book the glasses symbolize logic and inteligence
They use them as magnifying glasses to start fires.
Piggy's glasses symbolizes Knowledge and Power.
Piggy's glasses symbolise technology and science, they are also an outward indication of Piggy's intelligence. The glasses are the technology which provides the boys with fire. Piggy's spectacles are also the means by which Piggy is able to see clearly, both in reality and metaphorically.
Simon retrieved Piggy's glasses for him, after they had fallen onto the rocks as a result of Jack punching Piggy.
In Lord of the Files, the shell and the glasses come to symbolize the following; the shell (conch) symbolizes civilization and order, while the glasses symbolize civilized thought and the fire, which is the only way for them to get home. -student