The book is a metaphor and allegory for drug abuse and rehabilitation. The caterpillar is a metaphor for a drug-riddled man who has degenerated into a pot-head. The reason he's constantly hungry is since he constantly smokes pot. After a while, he starts becoming obese due to his constant use. The cocoon is a metaphor for rehab when he learns he needs to fix his issue and shelters himself from the world so he can heal in peace. The butterfly is a metaphor for the man after he's experienced sufficient rehab and has blossomed into a handsome, well-built and athletic man who can reach his full potential.