Golding uses the detail of Ralph's fingernails being chewed on to indicate Ralph's nervous state. For many people, Ralph included, fingernail chewing is a nervous tic, a way for him to cope with the stress of a situation. He gets nervous, so he chews his fingernails down. This detail is interesting, because it is also exactly the sort of bad habit that parents would frown upon. Had Ralph gotten a scolding from his mother for chewing his fingernails? He probably has; it is a childish habit, but one hard to break, and the fact that Ralph is reverting back to his old habits like nail-biting, and then later he mentions he will probably be "sucking [his] thumb next," suggests that Ralph is really stressed out; these habits are one way for him to diffuse all of his pent-up emotions. The notion of him sucking his thumb really suggests that he yearns for the comfort of his early childhood and the security of his parents.