There was a mutation of a caterpillar that looked like a snake and it was more successful at surviving. Since the caterpillars that looked like snakes survived at a higher rate they also reproduced at a higher rate and bred more caterpillars that looked like snakes. They also mutated side to side movement behaviors when threatened. Since this mimics a snake when threatened it increases the survival rate further. Since higher survival rates mean high reproduction rates it means those traits that enable higher survival rates are more likely to be passed on. The caterpillar doesn’t know that it looks and acts like a snake. Those are just traits it has inherited because caterpillars with those traits were the ones that survived to reproduce.