The Monarch’s wingspan ranges from 8.9–10.2 cm (3½–4 in.) The upper side of the wings is tawny-orange, the veins and margins are black, and in the margins are two series of small white spots. The fore wings also have a few orange spots near the tip. The underside is similar but the tip of the fore wing and hind wing are yellow-brown instead of tawny-orange and the white spots are larger.
The male has a black patch of androconial scales responsible for dispersing pheromones on the hind wings, and the black veins on its wing are narrower than the female’s. The male is also slightly larger.
A color variation has been observed in Australia, New Zealand, Indonesia and the United States as early as the late 19th century. Named nivosus by Lepidopterists, it is grayish white in all areas of the wings that are normally orange. Generally it is only about 1% or less of all monarchs, but has maintained populations as high as 10% on Oahu in Hawaii, possibly due to selective predation.
Like all insects the Monarch has six legs, however it uses only four of its legs as it carries its two front legs against its body.
The eggs are creamy white and later turn pale yellow. They are elongate and subconical, with approximately 23 longitudinal ridges and many fine traverse lines.
A single egg weighs about 0.46 milligrams (0.0071 gr), and measures about 1.2 millimetres (47 mils) high and 0.9 millimetres (35 mils) wide.
The caterpillar is banded with yellow, black, and white stripes. The head is also striped with yellow and black. There are two pairs of black filaments, one pair on each end of the body. The caterpillar will reach a length of 5 cm (2 in).
The chrysalis is blue-green with a band of black and gold on the end of the abdomen. There are other gold spots on the thorax, the wing bases, and the eyes.
In North America, the Monarch ranges from southern Canada to northern South America. It rarely strays to western Europe (rarely as far as Greece) from being transported by U. S. ships or by flying there if weather and wind conditions are right. It has also been found in Bermuda, Hawaii, the Solomons, New Caledonia, New Zealand, Australia, New Guinea, Ceylon, India, the Azores, and the Canary Islands.
The Monarch can be found in a wide range of habitats such as fields, meadows, prairie remnants, urban and suburban parks, gardens, and roadsides. It overwinters in conifer groves.
Monarchs are especially noted for their lengthy annual migration. In North America they make massive southward migrations starting in August until the first frost. A northward migration takes place in the spring. The monarch is the only butterfly that migrates both north and south as the birds do on a regular basis. But no single individual makes the entire round trip. Female monarchs deposit eggs for the next generation during these migrations.
By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests in the Mexican states of Michoacán and México. The western population overwinters in various sites in central coastal and southern California, United States, notably in Pacific Grove and Santa Cruz.
The length of these journeys exceeds the normal lifespan of most monarchs, which is less than two months for butterflies born in early summer. The last generation of the summer enters into a non-reproductive phase known as diapause and may live seven months or more. During diapause, butterflies fly to one of many overwintering sites. The generation that overwinters generally does not reproduce until it leaves the overwintering site sometime in February and March.