Lord of the Flies, by William Golding, is a symbolic novel; its characters are all English schoolboys who have been stranded on a tropical island. In the beginning of the novel, they are proper English schoolboys who hold meetings and follow some kind of order. Very quickly, however, they begin to lose their civility; by the end of the novel, Golding refers to them only as "savages."
In chapter eight, Jack (the eventual chief of the savages) and his hunters want to kill a pig so the tribe will have meat. Most of the other hunts in the novel are simply referenced, but Golding chooses to describe this one in great detail. Clearly the language (both literal and figurative) he chooses to use is similar to how one might describe a woman being raped.
Here, struck down by the heat, the sow fell and the hunters hurled themselves at her. This dreadful eruption from an unknown world made her frantic; she squealed and bucked and the air was full of sweat and noise and blood and terror. Roger ran round the heap, prodding with his spear whenever pigﬂesh appeared. Jack was on top of the sow, stabbing downward with his knife. Roger found a lodgment for his point and began to push till he was leaning with his whole weight. The spear moved forward inch by inch and the terriﬁed squealing became a highpitched scream. Then Jack found the throat and the hot blood spouted over his hands. The sow collapsed under them and they were heavy and fulﬁlled upon her.
This is just one more example of the boys' depravity on this island without any rules or authority. This scene and the language he uses prove Golding's point that living without societal constraints eventually leads to deviant and evil behavior, even in children.
It is the last line of the same paragraph which reminds us that this behavior is ugly and unnatural. Golding writes: "The butterﬂies still danced, preoccupied in the center of the clearing." Nature has not changed, but the evil nature of man has been revealed through these boys and this hunt. A few chapters later, the boys will commit several murders and nearly destroy themselves and the island in a conflagration--which, ironically, gets them rescued.
The effect of Golding's language on the reader is to create a sense of horror at the depravity of these once-proper schoolboys, and the butterfly is a reminder that what they are doing is not natural.