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Monarch butterfly
Monarch Butterfly on an Echinacea purpurea
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Lepidoptera
Superfamily:	Papilionoidea
Family:	Nymphalidae
Subfamily:	Danainae
Tribe:	Danaini
Genus:	Danaus
Species:	Danaus plexippus
Binomial name
Danaus plexippus (Linnaeus, 1758)

The butterfly species Danaus plexippus is commonly known as the monarch butterfly. It is perhaps the most well-known of all North American butterflies. Since the 19th century, it is also found in New Zealand, and has been known in Australia since 1871. [1] [2] In Australia it is also known as the wanderer butterfly. [3] In Europe it is resident in the Canary Islands and Madeira, and is found as a migrant in Russia, Azores, Sweden and Spain. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.5–12.5 cm (3.34 in–4.92in). (The viceroy butterfly has a similar size, color, and pattern, but can be distinguished by an extra black stripe across the hindwing.) Female monarchs have darker veins on their wings, and the males have a spot in the center of each hindwing [4] from which pheromones are released. Males are also slightly larger.

Migration

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. (In Australia, they make limited migrations in cooler areas, [5] but the blue tiger butterfly is better known in Australia for its lengthy migration. [6]) Female monarchs deposit eggs for the next generation during these migrations. By the end of October, the population of the Rocky Mountains migrates to the sanctuaries in the area of Angangueo, Ocampo, Zitácuaro and El Rosario in the Mexican state of Michoacán. The western population overwinters in various sites in central coastal 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. [7] 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. It is thought that the overwinter population may reach as far north as Texas and Oklahoma during the spring migration. It is the second, third and fourth generations that 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 circadian rhythm and the position of the sun in the sky.^[1]

Monarch butterflies are one of the few insects capable of making transatlantic crossings. They are becoming more common in Bermuda due to increased usage of milkweed 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 sighted as far east as Long Bennington. Monarchs can also be found in New Zealand. On the islands of Hawaii no migrations have been noted.

Reproduction

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. [8] 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 and involves the transfer of a spermatophore from the male to the female. Along with sperm, the spermatophore is thought to provide the female with energy resources that 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 complete metamorphosis. The Monarch goes through four radically different stages:

1. The eggs are laid by the females during spring and summer breeding months.
2. The eggs hatch, revealing worm-like larva, the caterpillars. The caterpillars consume their egg cases, then feed on milkweed, and sequester substances called cardenolides, a type of cardiac glycosides. During the caterpillar stage, monarchs store energy in the form of fat and nutrients to carry them through the non-feeding pupa stage.
3. In the pupa or chrysalis stage, the caterpillar spins a silk pad on a twig, leaf, etc, and hangs from this pad by its last pair of prolegs. It hangs upside down in the shape of a 'J', and then molts, leaving itself encased in an articulated green exoskeleton. At this point, hormonal changes occur, leading to the development of a butterfly. The chrysalis darkens (actually becomes transparent) a day before it emerges, and its orange and black wings can be seen.
4. The mature butterfly emerges after about two pupal weeks and hangs from the split chrysalis for several hours until its wings are dry (often in the morning). Meanwhile fluids are pumped into the crinkled wings until they become full and stiff. Some of this orangy fluid drips from the wings. Finally (usually in the afternoon) the monarch spreads its wings, quivers them to be sure they are stiff, and then flies in a circle and away, to feed on a variety of flowers, including milkweed flowers, red clover, and goldenrod.

Monarchs can live a life of twenty to eighty weeks in a garden having their host Asclepias plants and sufficient flowers for nectar. This is especially true if the flower garden happens to be surrounded by native forest that seems to be lacking in flowers.

Pictorial lifecycle

• 
  Monarch male showing its wings to attract a mate
• 
  Monarch butterfly mating
• 
  Monarch butterfly laying eggs
• 
  Monarch eggs on milkweed
• 
  An early instar monarch caterpillar
• 
  A late instar caterpillar feeding
• 
  Monarch caterpillar beginning pupation
• 
  Monarch butterfly chrysalis
• 
  Butterfly emerging from chrysalis
• 
  Adult monarch butterfly feeding on a Zinnia
• 
  Wintering monarch butterflies

Systematics

This butterfly is closely related to one or two very similar species which were formerly considered to form the Danaus (Danaus) subgenus (see Smith et al. 2005). One is the Jamaican monarch, Danaus cleophile, of Jamaica and Hispaniola.

The other is a butterfly which is almost indistinguishable from the monarch as an adult (the pupae are somewhat more different) and occurs south of the Amazonas river. This taxon, Danaus erippus ("Southern Monarch"), is often considered a subspecies of the monarch butterfly proper. Analysis of morphological, mtDNA 12S rRNA and cytochrome c oxidase subunit I, and nuclear DNA 18S rRNA and EF1 subunit α sequence data (Smith et al. 2005) indicates that it is better considered a distinct species.

However, as a species the southern monarch is only comparatively recently evolved. In all likelihood, its ancestors separated from the monarch's population some 2 mya, at the end of the Pliocene, when sea levels were higher and the entire Amazonas lowland was a vast expanse of brackish swamp that offered hardly any butterfly habitat (Smith et al. 2005).

Following the review of Smith et al. (2005), two subspecies are nowadays recognized:

• Danaus plexippus plexippus

  The migratory subspecies known from most of the USA

• Danaus plexippus megalippe

  The non-migratory subspecies which is found from Florida and Georgia southwards, throughout the Caribbean and Central America to the Amazon River. This has several local forms:

  • Danaus plexippus megalippe forma leucogyne
  • Danaus plexippus megalippe forma portoricensis
  • Danaus plexippus megalippe forma tobagi

Defense against predators

Monarchs are foul-tasting and poisonous due to the presence of cardenolide aglycones in their bodies, which the caterpillars ingest as they feed on milkweed. [9] Both forms advertise their unpalatability with bright colors and areas of high contrast on the skin or wings. This phenomenon is known as aposematism.

Monarchs share this defense with the even more unpleasant-tasting and similar-appearing viceroy butterfly, in an example of Müllerian mimicry. (Viceroys were at one time believed to be Batesian mimics of monarchs.)

Relationship with humans

The monarch is the state insect of Alabama,^[2] Idaho,^[3] Illinois,^[4] Minnesota,^[5] Texas,^[6] and the state butterfly of Vermont^[7] and West Virginia.^[8] It was nominated in 1989 as the national insect of the United States of America.

Many people like to attract monarchs by growing a butterfly garden. Others enjoy raising them for pleasure or for educational purposes. For migrating flocks, sanctuaries have been created at favorite wintering locations; Even tourism revenue is thus generated.

Threats

Recent illegal deforestation of the monarch's overwintering grounds have led to a drastic reduction in the butterfly's population. Efforts to classify it as a protected species and to restore its habitat are under way. Mexican authorities expected a significant increase in the monarch population in the 2005-2006 season.

Monarch butterflies are susceptible to Ophryocystis elektroscirrha, a protozoan which parasitizes them. It is present on the abdomen of adult butterflies and passed to their offspring when the spores rub off during egg-laying and are then ingested by the caterpillars. The effects of the parasite on Monarchs include decreased weight, shortened lifespan, weakened wings, rapid weightloss, or inability to eclose, though this varies between butterfly populations and parasite strains.

References

• Smith, David A. S.; Lushai, Gugs & Allen, John A. (2005): A classification of Danaus butterflies (Lepidoptera: Nymphalidae) based upon data from morphology and DNA. Zool. J. Linn. Soc. 144(2): 191–212. doi:10.1111/j.1096-3642.2005.00169.x (HTML abstract)

External links

Wikimedia Commons has media related to Danaus plexippus.

• Monarch butterfly facts and photos - NatureMapping Program
• Video of a monarch butterfly emerging from chrysalis
• Australian Museum fact sheet on monarch butterflies
• Critters of Calamvale Creek — a monarch tells its story
• Monarch Migration Maps
• Monarch Watch of the University of Kansas Entomology Department
• Michoacan Reforestation Fund
• USGS description of monarch butterfly
• MonarchHealth!
• Monarch Butterfly Trust NZ
• Monarch butterfly life cycle photographs
• More Than Monarchs