Monarch butterfly (Danaus plexippus)
The Monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the best known of allNorth American butterflies. Since the 19th century, it has been found in New Zealand, and in Australia since 1871, where it is called thewanderer. It is resident in the Canary Islands, the Azores, and Madeira, and is found as an occasional migrant in Western Europe and a rare migrant in the United Kingdom. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in). (Theviceroy butterfly is similar in color and pattern, but is markedly smaller, and has an extra black stripe across the hind wing.) Female monarchs have darker veins on their wings, and the males have a spot called the androconium in the center of each hind wing. Males are also slightly larger than female monarchs. The Queen is a close relative.
The monarch is famous for its southward late summer/autumn migration from the United States and southern Canada to Mexico and coastal California, and northward return in spring, which occurs over the lifespans of three to four generations of the butterfly. The migration route was fully determined by Canadian entomologists Fred and Norah Urquhart after a 38-year search, aided by naturalists Kenneth C. Brugger and Catalina Trail who solved the final piece of the puzzle by identifying the butterflies' overwintering sites in Mexico. The discovery has been called the "entomological discovery of the 20th century". An IMAX film, Flight of the Butterflies, tells the story of the long search by the Urquharts, Brugger and Trail to unlock the secret of the butterflies' migration. There is evidence that eastern North American populations of the monarch butterfly migrate to south Florida and Cuba.
Monarchs are especially noted for their yearly migration over long distances. In North America they make massive southward migrations starting in August until the first frost. There is a northward migration in the spring. The monarch is the only North American butterfly that migrates both north and south as the birds do regularly, but no individual makes the entire round trip. Female monarchs lay eggs for the next generation during these migrations. Painted Ladies also make a return migration from Africa to Northern Europe 
By the end of October, the population east of the Rocky Mountains migrates to the sanctuaries of theMariposa Monarca Biosphere Reserve within the Trans-Mexican Volcanic Belt pine-oak forests in theMexican states of Michoacán and México. The western population overwinters in various coastal sites in central and southern California, United States, notably in Pacific Grove, Santa Cruz, and Grover Beach.
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 nonreproductive phase known as diapause, which may last seven months or more. During diapause, butterflies fly to one of many overwintering sites. The overwintering generation generally does not reproduce until it leaves the overwintering site sometime in February and March.
The overwintered population of those east of the Rockies may reach as far north as Texas and Oklahoma during the spring migration. The second, third and fourth generations return to their northern locations in the United States and Canada in the spring. How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of the position of the sun in the sky and a time-compensated Sun compass that depends upon a circadian clock based in their antennae. New research has also shown these butterflies can use the earth's magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein sensitive to the violet-blue part of the spectrum. In presence of violet or blue light, it can function as a chemical compass, which tells the animal if it is aligned with the earth's magnetic field, but it cannot tell the difference between magnetic north or south. The complete magnetic sense is present in a single antenna.
Monarch butterflies are one of the few insects that can cross the Atlantic. They are becoming more common in Bermuda, due to increased use ofmilkweed as an ornamental plant in flower gardens. Monarch butterflies born in Bermuda remain year round due to the island's mild climate. A few monarchs turn up in the far southwest of Great Britain in years when the wind conditions are right, and have been seen as far east as Long Benningtonin Lincolnshire. In Australia, monarchs make limited migrations in cooler areas, but the blue tiger butterfly is better known in Australia for its lengthy migration. Monarchs can also be found in New Zealand. On the islands of Hawaii, no migrations have been noted.
Monarch butterflies are poisonous or distasteful to birds and mammals because of the presence of the cardiac glycosides contained in milkweed eaten by the larvae. The bright colors of larvae and adults are thought to function as warning colors. During hibernation, monarch butterflies sometimes suffer losses because hungry birds pick through them looking for the butterflies with the least amount of poison, but in the process kill those they reject.
One study examined wing colors of migrating monarchs using computer image analysis, and found migrants had darker orange (reddish-colored) wings than breeding monarchs.
Research also has overturned a prevailing theory that the migration patterns of the eastern and western populations are due to genetic reasons and that their genetic material was different. The American populations have been found to be distinct from the populations in New Zealand and Hawaii, but not from each other.
Mate pairing in Danaus plexippus does not depend on parasitism levels, but is affected by size, fluctuating asymmetry, and wing condition of females. Size affected mating occurs early in the mating season, where smaller females mate with smaller males and larger females mate with larger males. However, by the end of the mating season, larger females contain less spermatophoresthan smaller females. Mating females are more asymmetric than non-mating females, and forewing asymmetry plays a role in determining mate pairing. Studies suggest that large and symmetrical females are more attractive to males, but they are better at resisting male mating attempts. Larger males tend to mate with larger females because they are more likely to overcome the resistance of larger females. In addition, studies suggest that damaged wings decrease mating in females. Studies have failed to reveal correlations between male size or wing condition with male mating success. Successful males,however, mate on a greater proportion of days and are also more likely to be successful, ending in copulation. In addition, both females and males mate multiply. Spermatophore nutrients after mating are digested and used in female tissue. Because female Lepidoptera benefit from spermatophore-derived nutrients, there is a high frequency of multiple mating among female monarchs. In addition, females allowed to mate several times therefore laid more eggs than females who only mate once.
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 on either hind wing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs), 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, but uses the four hindlegs as it carries its two front legs against its body.
The eggs are creamy white and later turn pale yellow. They are elongated and subconical, with about 23 longitudinal ridges and many fine traverse lines. A single egg weighs about 0.46 mg (0.0071 gr), and measures about 1.2 mm (47 mils) high and 0.9 mm (35 mils) wide.
The caterpillar is banded with yellow, black, and white stripes. The head is also striped with yellow and black. Two pairs of black filaments are seen, one pair on each end of the body. The caterpillar reaches 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. Other gold spots occur on the thorax, the wing bases, and the eyes.
The monarch can be found in a wide range of habitats, such as fields, meadows, prairie remnants, urban and suburban parks, gardens, trees, and roadsides. It overwinters in conifer groves
The mating period for the overwinter population occurs in the spring, just prior to migration from the overwintering sites. The courtship is fairly simple and less dependent on chemical pheromones in comparison with other species in its genus. Courtship is composed of two distinct stages, the aerial phase and the ground phase. During the aerial phase, the male pursues, nudges, and eventually takes down the female. Copulation occurs during the ground phase, where the male and female remain attached for about 30 to 60 minutes. Only 30% of mating attempts end in copulation, suggesting that females have methods to avoid unwanted matings. Differences in female ability to resist mating affect pairing patterns. A spermatophore is transferred from the male to the female. Along with sperm, the spermatophore is thought to provide the female with energy resources to aid her in carrying out reproduction and remigration. The overwinter population returns only as far north as they need to go to find the early milkweed growth; in the case of the eastern butterflies, that is commonly southern Texas. The life cycle of a monarch includes a change of form called completemetamorphosis. The monarch goes through four radically different stages: