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Stages
A butterfly starts out as an egg, and hatches out as a caterpillar. The caterpillar will grow and become a pupa, and then emerge as an adult. The Butterfly Site states that this metamorphosis may take anywhere from a month to a year, depending upon the species. Some butterflies such as the monarch will migrate to warmer spots in the winter, returning the following season to lay their eggs.
The Egg
Butterflies lay their eggs on plants that are the normal food of the larvae, so the caterpillars will not have to travel far in search of food. The eggs are small and may be either round or cylindrical in shape, depending upon the species that laid the egg. The egg is transparent, and a caterpillar is seen inside the egg upon close examination.
Larval Stage
The second stage in a butterfly's life is the larval stage. When the egg hatches, a young caterpillar emerges. The caterpillar will spend this whole portion of its life eating, and will rapidly grow in size. The skin of the caterpillar is its exoskeleton, which will stretch but will not grow with the caterpillar. A caterpillar will shed this skin a number of times as it grows and gets too large for its skin.
The Chrysalis
A caterpillar will form a chrysalis (a firm case or cocoon) after it has eaten sufficiently and grown to the proper size. This stage of the butterfly's life is also known as the pupa. The caterpillar will attach its tail to a twig and shorten its body. The exoskeleton will harden, while the caterpillar undergoes changes inside this chrysalis.
Metamorpohosis
While it is in the chrysalis, the changes the caterpillar undergoes are dramatic. The chrysalis will appear to change color, but it is actually transparent, with the pupa inside undergoing the color changes. The body of the fat caterpillar slims down significantly, and wings form. Six elongated legs will grow, along with a distinct head and antennae.
Butterfly Emergence
A miniature butterfly can be seen within the chrysalis as metamorphosis progresses. When it is mature, the chrysalis will split and an adult butterfly will emerge. The butterfly's wings are folded tightly upon emergence, and it will remain with the chrysalis while it pumps fluid into the veins in its wings. This causes the wings to unfold into the flat wings of an adult.