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Monarch Life Cycle

Bob Parks closeup photo of Monarch eggs. Eggs
Monarch eggs are about the size of a pinhead, shiny, and yellowish-white with longitudinal ridges. The eggs are found on milkweed plants, usually on the underside of a leaf. The eggs hatch in three to four days.
  Larva
The larva (caterpillar) that emerges from the egg is small, about 0.2 inch (0.5 cm), with pale yellow, black, and white stripes. The Monarch larva eats only milkweed leaves. As the larva grows, it molts (sheds its exoskeleton) five times. The larva has a head with chewing mouthparts and six pairs of simple eyes. Each of the three thoracic segments has a pair of true legs while there are usually five pairs of prolegs (false legs) on the abdominal segments. The larval stage lasts about two weeks. Toxic substances in the milkweed plant are absorbed by the larva, providing it with protection from predators.
Pupa
When ready to pupate, the Monarch larva spins a pad of silk on a twig or other suitable item. It then splits its exoskeleton, wiggles free of the larval skin, and attaches the cremaster—a hooked appendage at the end of the abdomen—to the silk pad. There, the shiny, green pupa (chrysalis) with gold dots will hang for about two weeks. This is a transformational stage where the larva changes into an adult. In this photo, the chrysalis has become transparent. The adult butterfly will soon emerge.
Queen butterfly -- follow link to more information (photo by Bob Parks) Adult
Bob Parks photo of adult
The emerging adult butterfly rests for several hours as its wings dry and stiffen. Male Monarchs may be distinguished from females by a black patch of scales near the inner margin of the hindwing. (The male Monarch is pictured here.)

Adults feed on nectar from milkweed flowers as well as many others. The toxins ingested by the larva continue to provide the adult protection from predators. The Monarch's bright colors serve to warn predators of its toxicity. Other non-toxic species may have similar bright colors enabling them to avoid predation—a phenomenon known as mimicry.

The Viceroy butterfly has often been cited as a Monarch mimic. Believed to be a non-toxic species, it was thought that the Viceroy's bright coloration (similar to the Monarch) protected it from predation. There is now some evidence that the Viceroy is itself a noxious species. Perhaps it is not a true mimic, but simply possesses "warning" colors in its own right. Viceroys do not belong to the same family as Monarchs and are not found in San Diego County.

Looking somewhat like the Monarch, the Queen is also a milkweed butterfly. Its bright colors serve to alert predators of its unpalatable, noxious taste. Queens are found in San Diego County. (See Field Guide pages for Monarch and Queen.)

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Life Span and Migration

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