In Chapter Four of Lord of the Flies, Roger restrains himself from pelting little Henry who sits on the beach's edge playing with the small crabs that come ashore. As Golding writes,
Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger's arm was conditioned by a civilization that knew nothing of him and was in ruins.
The irony of the "sign from the world of grown ups," the body of the dead parachutist, exists in its horrific representation of the present world of adults in which a world war is being waged. For, the society designed to protect and provide policemen and laws now is, indeed, in ruins as it is involved in destroying human life itself. Thus, the parachutist is the same as the pigs that Jack and the hunters so brutally destroy. In short, the world of adults that is the hope of the boys for rescue is itself as savage as Jack and the hunters have become. Symbolic of this fact, the parachutist becomes equated with the beast, and unconsciously, the boys sense the evil that is man.
As Ralph is hoping that if only they could get a "grown-up sign" to show them what grown-ups would do in a specific situation, unknown to the kids, that night an airplane was shot down. As a result, the pilot that was flying the plane dies, and his dead body ejected from the airplane, landing in the middle of the forest with his parachute wrapped around him.
This is very ironic because, first of all, there goes another dead adult. Second, this adult could have been the salvation of these children, as a pilot and, obviously, as a soldier of some kind. Third, the children could not see him. Could it be that the children were so submerged into their new little civilization that they were unable to distiguish chaos? There is a lot to be said in these chapters, but they are the most rich ones in the story.