Butterflies, Lepidoptera, undergo complete metamorphosis. They begin as eggs, hatch into caterpillars, then hang in chrysalises and transform into the adult stage, the imago. A caterpillar is utterly different from an adult. It eats different food, inhabits a different niche, and looks so different that it is only genetically recognizable as the same creature.
A butterfly egg contains genetic material to produce radically different stages of life. While it is a caterpillar, a larva, all the genetic directions for wings and reproduction are held latent, unexpressed. They will begin to manifest themselves only when the caterpillar enters its chrysalis, and begins to change into its true adult form.
Butterfly eggs are tough. They can withstand freezing, and many can withstand corrosive conditions. Some have knobs or sharp points, and many have elaborate textures or patterns.
When certain species of butterflies hatch, they eat their eggshells. If the eggs of other butterflies are near, they eat those too. However, other species leave their eggs in mounds or towers, and the caterpillars that chew their way out of these eggs will not eat the rest of the eggs.
Caterpillars are eating machines. They eat a simple diet, often from only one particular kind of plant. Generally, they will starve rather than eat another kind of leaf. Their jaws are tough and well adapted to grinding leaves. They eat voraciously, growing fast.
They have simple eyes. Some species can barely distinguish light from darkness. Their short antennae carry their sense of touch. Their sense of taste is acute.
They have a spinneret, a silk-making gland, on their lower lip. They extrude two compounds that combine to form their thread, often colored by their diet. They move along this line, clinging to it as they go. If a bird attacks, a caterpillar may drop along its thread, to disappear from view, and eat its way back up when the danger has passed.
Caterpillars are often camouflaged with defensive patterns and colors. Some disguises match the local foliage. Others announce that the insect is poisonous or otherwise dangerous, with bright colors.
Some caterpillars have stinging hairs and others retain poisons from their food. The Monarchs, for example, live on milkweed, which makes them toxic, or at least bad tasting. They warn off potential predators with colorful notice of these defenses. Some defenseless caterpillars, on the other hand, disguise themselves as dangerous. Some, when stressed, give off a noxious smell.
Caterpillars have their skeletons on the outside, like any insect. Therefore, they must molt out of their confining skins to grow. Each growth period between a molt is called an instar, and caterpillars may have several. After each molt, the caterpillar abruptly appears a size larger. Its appearance may also change somewhat with each molt. When a caterpillar comes to its last molt, hormones signal that it is time to pupate.
Generally, butterflies pupate in a chrysalis, while moths spin a cocoon. The difference is silk. Most moths spin silk around themselves, while most butterflies pupate in only their skin. As they pupate, most chrysalises hang motionless from silk harnesses. Within its pupal skin, the insect changes rapidly.
The chrysalis is often disguised. Some swallowtail chrysalises resemble leaves or chunks of bark. A chrysalis may hang for a week, or many months, depending on the species and the season.
Long flexible legs develop within, for landing gear. Mouth parts change so the butterfly can suck up nectar or other liquids. Soft wings develop, and reproductive organs the caterpillar did not need. The muscle structure changes to adapt to the life of a flier.
The larval cells die. The adult cells that had been held back take over. To power their growth and division, they feed on the larval cells.
When the metamorphosis is finished, the butterfly molts one last time. It sheds it pupal skin. Now it emerges, but it is soft and vulnerable. It climbs a twig or blade of grass, its wings folded and drooping. Slowly, the wings unfurl, drying and firming in the air. It may take two hours for them to firm. Then the butterfly beats them two or three times, and flies.
It will not grow anymore, but it will fly. Butterfly wings are generally covered with glinting overlapping scales that carry its delicate coloring. Its mouth is a long tube, the proboscis, carried curled in a spiral beneath its head. Butterflies suck up nectar, or soft rotting fruit. Their proboscis uncurls as a reflex when they taste nectar. Their taste organs are located on their feet, and go into action when they land on ripe flowers or fruit.
Many butterflies have antennae that end in a knob or club, while many moths have feathery antennae. (However, this is an unreliable way to tell moths and butterflies apart, as are most ways.)
Male butterflies use their antennae to locate females by scent, or rather by pheromones, chemical signals that trigger the mating response. Males can detect them miles away.
Females sniff out plants where they will deposit their eggs. They also search with their faceted eyes. Both genders have elaborate compound eyes, far more sophisticated than those of the caterpillar.
On average, counting caterpillar, pupa, and adult forms, butterflies live 5 to 8 weeks. It is not long, though it is a lifetime. Butterflies that overwinter in diapause, a state of dormancy, may live longer. Overall, though, an adult butterfly lives about two weeks on average, perhaps longer if it flies in cooler weather.
A caterpillar eats and grows, and stores up energy for the adult form. The pupa transforms into an imago. The adult leaves the eggs. The genetic information within the sturdy eggs makes possible the metamorphosis that leads to the delicate flight of the butterfly.