In Lord of the Flies chapter 5, why is the beast from the water and ghosts introduced in this particular chapter and why not before or later?

In Chapter 5 of "The Lord of the Flies," Ralph feels the need to call for a meeting since responsibilities are being neglected.  During this meeting, the boys discuss their fears.  Phil, a littl'un says that he has had dreams of "snake-like" things. Others say that they have seen the beast.  But Jack contends that he would have seen this beast since he has covered the island.  Percival then says that the beast comes out of the sea.  To this Maurice relates a tale of squids eating whales whole, but some others reject this idea. 

Soon, the area is filled with argument over the conjecture of Maurice.  Only Simon knows that the beast does not issue from the sea; he knows that the beast in within the boys, but Simon "became inarticulate in his effort to express mankind's essential illness and his attempt to tell the boys that the beast is their own evil "fell about him in ruins; the laughter beat him cruelly and he shrank away defenseless to his seat." Their misunderstanding leads them to say, "Maybe he means it's some sort of ghost," resorting to the superstitious in order to explain.

Piggy, the voice of reason, declares, "I don't believe in no ghosts--ever!"  Ralph says there is too much talk of ghosts.  He decides to let the boys vote as to who believes in ghosts.  He counts the hands and replies, "I see":

The world, that understandable and lawful world, was slipping away.  Once there was this and that: and now--and the ship had gone.

Until this point, Golding has described the idyllic paradise in which the boys live. Now, after being without parents and society, the boys' fears take hold of them and they see "ghosts" and a beast coming from the sea because they cannot deal with their increasing aloneness on this island. 

Now, Jack vies for leadership with Ralph and shouts against Ralph as he reminds the boys of the rules:

Bullocks to the rules!  We're strong--we hunt!  If there's a beast, we'll hunt it down!  We'll close in and beat and beat and beat--!"

 But the boys' fears remain as a "thin wail oout of the darkness chilled them and set them grabbing for each other."  In the next chapter, "a sign came down from the world of grown-ups, though at the time there was no child awake to read it."  And, then, the dead parachutist appears as a sign "from the world of grown-ups, though at the time there was no child awake to read it."  And, so, the little boys no longer see "ghosts."  Instead, they become swept into the activity of Jack:  hunting the pigs.  Later, SamnEric contend that they have seen the beast that has chased them and clawed them. So, now the beast is not of the sea, nor is he a ghost.

The beast has been a product of many of the boys' imaginations, so it has changed from a thing of the sea to red and gruesome creature flapping in the wind.  Now that the beast begins to have a form, all thoughts of it as a "ghost" are dismissed.