In order to turn into their adult form, butterflies must pupate, or go into a cocoon. However, before they do this, they exist for a number of weeks in their larval form and prior to that they grow inside eggs. Butterfly larvae are also known as caterpillars. Different species have different appearances, but they all look relatively similar.
Before butterflies reach their larval phase, they exist as eggs. The adult butterfly lays her eggs so they attach to the leaf of a plant. These eggs vary in appearance according to the species of butterfly. However, they're always very small and can be either round, oval or cylindrical in shape. If you look extremely closely at some types of butterfly eggs, you can see a tiny caterpillar starting to grow.
Once the eggs have hatched, a larval butterfly -- or caterpillar -- will emerge. They start out very small, but grow quickly until it's time to pupate. Different species of butterfly produce caterpillars of various colors and sizes, but all of them have the same basic form. They're roughly cylindrical in shape, with a head, thorax and thoracic legs at the front end, and an abdomen, abdominal prolegs and anal prolegs at the back end. Some caterpillars are smooth, whereas others are covered in hairs.
The majority of caterpillars are strict herbivores. In fact, many have an extremely limited diet and will only eat one kind of leaf. In these species, the mother butterfly will ensure she lays her eggs on the one type of plant her babies will eat. As caterpillars need to grow significantly, they spend most of the day eating leaves with their strong mandibles. As they get bigger, their exoskeleton doesn't grow with them, so they have to shed it several times before they reach the cocoon phase.
If a caterpillar hopes to stay safe long enough to pupate and become a butterfly, it will have to avoid a number of threats. Caterpillars have lots of natural predators, such as birds, wasps, reptiles and small mammals. They're seen as a pest by some people as they eat plant leaves, so they're also under threat of being poisoned by insecticides or otherwise killed by humans.