Answer
A synecdoche is a figure of speech in which a part is made to represent a whole or vice versa.
Explanation:
In Lord of the Flies, the pig's head is a perfect and frequent example of a synecdoche.
Piggy is a perfect and frequent example of a synecdoche. Piggy is not his real name, but, because of his short stature and fat, he is called Piggy by the other boys.