The butterfly begins life as an egg, emerges as a caterpillar, and then undergoes a complete change in body form during development.
The wormlike butterfly larva, or caterpillar, looks nothing like a winged adult. After hatching from the egg, the young caterpillar spends most of its time eating leaves and gaining weight. The caterpillar undergoes several molts of its skin until it becomes full grown and has accumulated enough body mass to carry it through the entire life cycle, including the adult phase.
The caterpillar then enters the pupal stage, when it neither feeds nor moves. From the outside, it appears as though the pupa, also known as the chrysalis, is resting. In reality, though, the larval tissues completely break down and reorganize within the pupal skin. What emerges from the chrysalis is a fully formed adult—a butterfly.
The butterfly's wings are initially soft and shriveled, but they expand and harden within a few hours. The butterfly then takes flight to pursue its main adult activities, mating and reproduction. It has no time to waste—adults of most species live for only a few weeks.
The butterfly begins its life as an egg about the size of the head of a pin. Adult female butterflies usually lay their eggs on plant leaves or stems. Some place the eggs in protected locations—on the undersides of leaves, for example—where wasps and other predators are less likely to find them.
As the time of hatching nears, a week or two after the egg is laid, the eggshell darkens and becomes almost transparent. At this stage you can see a tiny but fully formed caterpillar moving inside. Finally, the larva chews through the eggshell and emerges into the world.
The butterfly larva, or caterpillar, spends most of its time eating, digesting its food, and growing.
Caterpillars consume huge quantities of leaves—and they are very specific about which plants they will eat. Most will eat plants from only a single species or genus, called the host plant for that caterpillar; it will die rather than feed on others.
The larva repeatedly outgrows its skin, which splits and is shed. At the end of its growth period, the caterpillar stops eating and finds a good place to molt into the next stage. It spins a small pad of silk and attaches itself to it, hanging upside down, immobile. The larval skin then splits one last time, revealing the pupa.
Inside the skin of the pupa, or chrysalis, the most dramatic part of the metamorphosis takes place.
During this stage of metamorphosis, which usually takes from two weeks to several months, the larval tissues completely break down and reorganize. The outlines of adult features—the wings, eyes, tongue, antennae, and body segments—can be seen on the surface of the pupal skin.
When the is fully formed, the pupal case splits and the butterfly emerges. The butterfly first expels its meconium, metabolic waste products that have accumulated during the pupal stage. It then expands its shriveled wings—by pumping them full of blood—before flying off.
The primary function of the male butterfly is to find a female. The male butterfly uses vision to locate a female of his own species, then lures her with chemicals called pheromones, produced by his scent glands. Some species also perform elaborate courtship flights.
Once the female has mated, she must lay her fertilized eggs on the appropriate larval host plant. To find the host plant—an amazing feat, given the tremendous diversity of plants in the butterfly's surroundings—the females rely on vision and a highly tuned ability to detect plant chemicals.