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The monarch butterfly (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae. It may be the most familiar North American butterfly. The monarch butterfly is not currently listed under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) or protected specifically under U.S. domestic laws. Its wings feature an easily recognizable orange and black pattern, with a wingspan of 8.9–10.2 cm (3½–4 in). (The viceroy 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 eastern North American monarch population is notable for its southward late summer/autumn migration from the United States and southern Canada to Mexico, covering thousands of miles. The western North American population of monarchs west of the Rocky Mountains migrate to California.
The monarch was originally named by Linnaeus in his Systema Naturae of 1758 and it was placed in the genus Papilio. In 1780, Jan Krzysztof Kluk used the monarch as the type species for a new genus; Danaus.
The monarch is closely related to two very similar species that formed the Danaus (Danaus) subgenus before 2005. The first is the Jamaican monarch (D. cleophile) from Jamaica and Hispaniola. The second is the southern monarch (D. erippus), of South America south of the Amazon river. The southern monarch is almost indistinguishable from the monarch as an adult, though the pupae are somewhat different, and is often considered a subspecies of the monarch proper. But analysis of morphological, mtDNA 12S rRNA, cytochrome c oxidase subunit I, nuclear DNA 18S rRNA and EF1 subunit α sequence data by Smith et al. (2005) indicates it is better considered a distinct species. The separation of the monarch and southern monarch is comparatively recent. In all likelihood, the ancestors of the southern monarch separated from the monarch's population some 2 mya, at the end of the Pliocene. At the time, sea levels were higher and the entire Amazonas lowland was a vast expanse of brackish swamp that offered hardly any butterfly habitat.
Following the review of Smith et al. (2005), two subspecies of the monarch are recognized:
Smith et al. did not take up Danaus plexippus nigrippus in their studies. According to Hay-Roe et al. (2007), this taxon is still recognised as a subspecies:
In Homeric Greek plexippos (πληξιππος) means "one who urges on horses", i.e. "rider or charioteer". In the 10th edition of Systema Naturae, at the bottom of page 467, Linnaeus wrote that the names of the Danai festivi, the division of the genus to which Papilio plexippus belonged, were derived from the sons of Aegyptus.
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.
Since the 19th century, it has been found in New Zealand, and in Australia.  It is resident in the Caribbean, Canary Islands, the Azores, and Madeira, Portugal, Spain and is found as an occasional migrant in Western Europe and a rare migrant in the United Kingdom. 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 US ships or by flying there if weather and wind conditions are right. It has also been found in Bermuda, Cook Islands, Hawaii, the Solomons, New Caledonia, New Zealand, Australia, New Guinea, Ceylon, India, Nepal, the Azores, and the Canary Islands.
The eastern population migrates hundreds to thousands of miles to overwintering sites in Mexico. Southward migrations start in August and end at the first frost. There is a northward migration in the spring. The eastern population migrates both north and south on an annual basis. But no individual makes the entire round trip. Female monarchs lay 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 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.
The North American Western and Eastern populations (D. plexippus) migrate to established overwintering spots each autumn. In one study monarchs released during the fall migration from Albuquerque, New Mexico were found overwintering in California and in Mexico. The same study tested the ability of commercially bred monarchs (to the 9th generation) to migrate to overwintering areas. Flight navigational patterns may 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. These populations may use the earth's magnetic field for orientation. The antennae contain cryptochrome, a photoreceptor protein sensitive to the violet-blue part of the light spectrum. In the presence of violet or blue light, it can function as a chemical compass. Studies demonstrate that eastern and western populations do not use an internal ‘map’ to navigate to overwintering locations but instead are guided by a ‘compass’ which compels them to migrate in a southwest direction. This southwest directional migration is affected by large geographical features like the Rocky Mountains and The Gulf of Mexico.
Monarch butterflies can and have crossed the Atlantic. They are becoming more common in Bermuda and Spain, due to increased use of milkweed as an ornamental plant. Monarch butterflies in Bermuda and Spain do not migrate. Butterflies sometimes appear in Great Britain. In Australia, monarchs make limited migrations in cooler areas, On the islands of Hawaii, no migrations have been noted. The Southern Monarch, D. erippus migrates along the eastern edge of the Andean mountains in the autumn in Bolivia and Peru.
One study examined wing colors of migrating monarchs using computer image analysis, and found migrants had darker orange (reddish-colored) wings than breeding monarchs.
The monarch is not limited to forest sanctuaries but can found in agricultural fields and pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides. The eastern North American overwinters in Mexican conifer groves.
Although larvae eat only milkweed, adult monarchs feed on the following nectar plants:
Males also take in moisture and minerals from damp soil and wet gravel, a behavior known as mud-puddling. The monarch has also been noticed puddling at an oil stain on pavement. Adult butterflies are also attracted to the liquids of foods we consume; they will drink mushy slices of bananas, oranges, and watermelon.
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 than 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 complete metamorphosis. The monarch goes through four radically different stages:
Monarchs can live two to eight 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 forests that lack flowers.
Reproduction does not appear to be influenced by parasite levels. Instead it affected by size, fluctuating asymmetry, and wing condition of females. By the end of the mating season, larger females contain fewer spermatophores. Mating females are more asymmetric than non-mating females which plays a role in determining mate pairing. Females often resist male mating attempts. Studies suggest that damaged wings decrease mating in females. Males who are fit are more likely to mate a greater proportion of days and are also more likely to achieve copulation. Both females and males typically mate more than once. Spermatophore nutrients are absorbed and used in egg production. Females that mate several times laid more eggs than females who only mate once.
Male monarchs produce spermatophores, a sperm sac embedded in a gelatinous body, from accessory gland secretions. Since male monarchs are highly polyandrous and females store sperm, there is need for sperm competition, selecting for males that gain sperm precedence. The spermatophore size of males increases with increasing time between matings, and larger spermatophores delay female re-mating. Therefore, by waiting to re-mate, males increase their sexual competitiveness, ultimately increasing the number of ova fertilized by their sperm.
In addition, the production of large spermatophores could also benefit the females because spermatophore constituents may be used by females to affect the amount of offspring produced or the quality of offspring produced. By increasing female reproductive success, male reproductive success also increases. Studies have shown that the contents of Lepidopteran spermatophores are incorporated into the eggs and somatic tissue of females. Therefore, an increase in spermatophore size also increases the fecundity of female monarchs. Sperm from males that produce larger spermatophores could also fertilize more female's eggs without increasing her lifetime reproductive success.
Sperm precedence patterns were also observed, where second-male sperm precedence was more common than first or no-male sperm precedence. The advantage of second-male sperm suggests that the incoming sperm pushes back previously existing sperm, resulting in different layers of sperm from different males. However, second-male sperm precedence is rarely complete, suggesting that the sperm from the first male remains in the fertilization set, acting as a barrier to block sperm from other males.
Monarch butterfly laying eggs on Asclepias curassavica 'Silky Gold'
Monarch eggs on milkweed
An early instar monarch caterpillar
Monarch butterfly in Santa Barbara California
Adult monarch butterfly feeding on a Zinnia
The host plants used by the monarch caterpillar include:
In both caterpillar and butterfly form, monarchs use a bright display of contrasting colors to warn potential predators of its undesirable taste and poisonous characteristics. This aposematic behavior is common among many insects, amphibians, and mammals alike. Additionally, monarchs are physically similar to the viceroy butterfly, exhibiting a classic case of mimicry.
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. By ingesting a large amount of plants in the genus Asclepias, primarily milkweed, monarch caterpillars are able to sequester cardiac glycosides, or more specifically cardenolides, which are steroids that act in heart-arresting ways similar to digitalis. It has been found that monarchs are able to sequester cardenolides most effectively from plants of intermediate cardenolide content rather than those of high or low content.
Additional studies have shown that different species of milkweed have differing effects on growth, virulence, and transmission of parasites. One specific species (Asclepias curassavica) appears to reduce the proportion of monarchs infected by parasites. There are two possible explanations for the positive role of A. curassavica on the monarch caterpillar. The first is that A. curassavica promotes overall monarch health to boost the monarch’s immune system. A second theory is that A. curassavica has a direct negative effect on the parasites.
After the caterpillar becomes a butterfly, the toxin shift to different parts of the body. Since many birds attack the wings of the butterfly, having three times the cardiac glycosides in the wings leaves predators with a very foul taste, and may prevent them from ever ingesting the body of the butterfly. In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.
Monarch toxins are pharmacologically similar to digitalis and produce extremely similar results in experimental settings. In the wild, the toxins cause many birds to experience intense discomfort and vomiting. Many birds find monarchs unappetizing and quickly begin recognizing their distinct colors and avoiding them as food sources.
Monarchs share the defense of noxious taste with the similar-appearing viceroy butterfly in what is perhaps one of the most well-known examples of mimicry. Though long purported to be an example of Batesian mimicry, the viceroy is actually reportedly more unpalatable than the monarch, making this a case of Müllerian mimicry.
The monarch is the state insect of Alabama, Idaho, Illinois, Minnesota, Texas, Vermont, and West Virginia. It was nominated in 1990 as the national insect of the United States of America. but the legislation did not pass.
Monarchs can be attracted by cultivating a butterfly garden with specific milkweed species and nectar plants. Efforts are underway to establish these Monarch Waystations. Monarchs are raised as a hobby and for educational purposes. Butterfly farmers raise Monarchs and ship them to individuals and organizations to be released at a wedding or funeral, for example. The release of captive bred Monarchs remains controverial.
Some organizations, such as the Cape May Bird Observatory, have monarch identification tagging programs. Plastic stickers are placed on the wing of the insect with identification information. Tracking information is used to study their migration patterns, including how far and where they fly.
Although monarchs feed on milkweed, variations in the quantity of cardiac glycosides exist between species, individuals, and even parts of the host plant. The levels of toxins in adult monarchs reflect the levels in their host plants. This means some monarchs are not foul-tasting, but are Batesian or automimics. Some species of predators have learned to measure the toxins by taste and reject butterflies with high cardiac glycosides contents, eating only the ones with low contents.
Monarchs also contain cardiac glycosides in their bodies from the Asclepias plants the caterpillars eat. Overwintering monarchs in Mexico are often preyed upon by Black-headed Grosbeaks, which are immune to that toxin. Other birds, such as orioles and jays, have learned to eat only the thoracic muscles and abdominal contents because these contain less poison than the rest of the body. Some mice are also able to withstand large doses of the poison. Over time, overwintering adults become less poisonous, thus making them more vulnerable to predators. In Mexico, about 14% of the overwintering monarchs are eaten by birds and mice.
In the butterfly, the cardiac glycosides are concentrated in the abdomen and wings. Some species of predators differentiate these parts and consume only the most palatable ones. Bird predators include brown thrashers, grackles, robins, cardinals, sparrows, scrub jays and pinyon jays.
Another predator of the monarchs is the Chinese mantid (Tenodera sinensis). Chinese mantids have been shown to have adapted to handle monarch caterpillars to reduce poisonous effects. Chinese mantids are capable of identifying monarch larvae and approach them in a different manner than other caterpillars. Upon encountering a monarch caterpillar, Chinese mantis will chew open the integument to let the gut fall out. Once the gut is removed, they will proceed to consume the rest of the body. This is effective as monarch caterpillars have much higher levels of cardenolide content in their guts than in the rest of their bodies. By practicing this method of ingestion, Chinese mantids are able to avoid the gut which contains high levels of milkweed particles. This practice has also been documented in some other predators as well.
Several birds have also adapted various methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides. The oriole is able to eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them. The grosbeak, on the other hand, has adapted the ability an insensitivity to secondary plant poisons which allows it to ingest monarchs without vomiting. As a result, orioles and grosbeaks will periodically have high levels of cardenolides in their bodies, and they will be forced to go on periods of reduced monarch consumption. This cycle of predation effectively reduces the potential predation of monarchs by 50 percent and indicates that monarch aposematism has a legitimate purpose.
On Oahu, a white morph of the monarch has emerged. This is because of the introduction, in 1965 and 1966, of two bulbul species, Pycnonotus cafer and Pycnonotus jocosus. They are now the most common insectivore birds, and probably the only ones preying on insects as large as the monarch. Monarchs in Hawaii are known to have low cardiac glycoside levels, but the birds may also be tolerant of the chemical. The two species hunt the larvae and some pupae from the branches and undersides of leaves in milkweed bushes. The bulbuls also eat resting and ovipositing adults, but rarely flying ones. Because of its colour, the white morph has a higher survival rate than the orange one. This is either because of apostatic selection (i.e. the birds have learned the orange monarchs can be eaten), because of camouflage (the white morph matches the white pubescence of milkweed or the patches of light shining through foliage), or because the white morph does not fit the bird's search image of a typical monarch, so is thus avoided.
Parasites include the tachinid flies Sturmia convergens and Lesperia archippivora. Lesperia-parasitized larvae complete their moult and suspend, but die before pupation. At that time, one white maggot comes out of the larvae, suspended by a silken thread. The maggot then forms a brown pupa on the ground.
The bacterium Micrococcus flacidifex danai also infects the larvae and causes “black death”. As usual, just before pupation, the larvae migrate to a horizontal surface. They die a few hours later, attached only by one pair of prolegs, with the thorax and abdomen hanging limp. The body turns black shortly after. The bacterium Pseudomonas aeruginosa has no invasive powers, but causes secondary infections in weakened insects. It is a common cause of death in laboratory-reared insects.
The protozoan Ophryocystis elektroscirrha is another parasite of the monarch. It infects the subcutaneous tissues and propagates by spores formed during the pupal stage. The spores are found over all of the body of infected butterflies, with the greatest number on the abdomen. These spores are passed, from female to caterpillar, when spores rub off during egg-laying, and are then ingested by caterpillars. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but probably occur at low frequencies in nature. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.
The black swallow-wort is problematic for monarchs in North America. Monarchs lay their eggs on these relatives of native milkweeds because they produce stimuli similar to milkweed. Once the eggs hatch, the caterpillars are poisoned by the toxicity of this invasive plant from Europe.
The alarming decrease in the monarch butterfly population has been linked to the decrease in the milkweed plant (Asclepias)—a primary food for monarchs—from herbicide use in the butterfly’s reproductive and feeding areas. The destruction of common milkweed has effectively eliminated the food source from most of the habitat monarchs used to use. Common milkweed is suseptible to the use of herbicides. Varietals do exist, however, (see Human Interactions) that can be successfully planted in gardens and other areas to help mitigate habitat loss in the wild.
Genetically modified crops (GMCs), specifically corn and soybeans, now enable farmers to use herbicides to selectively eliminate milkweed that previously grew between the rows of food crops. These crops are genetically modified to be resistant to the effect of the herbicide glyphosate. The increased planting of glyphosate-resistant crops has had the side of effect of reducing milkweeds, which has been correlated with the decline in Monarch populations between 1999 and 2010. Chip Taylor, director of Monarch Watch at the University of Kansas, said the Midwest milkweed habitat "is virtually gone" with 120–150 million acres lost.
The area of forest occupied by overwintering monarch butterflies in Mexico reached its lowest level in two decades in 2013. According to a survey carried out during the 2012–2013 winter season by the WWF-Telcel Alliance, and Mexico’s National Commission of Protected Areas (CONAP), the nine hibernating colonies occupy a total area of 2.94 acres of forest—representing a 59% decrease from the 2011–2012 survey of 7.14 acres.
The same survey in 2012-2013 showed the decline is continuing. There were only seven colonies occupying 0.67 hectares (1.66 acres), the third consecutive record low since record-keeping began in 1995-1996. It represents a 44% decrease from the previous year, a 76% decrease from 2011-2012 and a 92% decrease compared to the 1996-1997 count.
Climate variations during the fall and summer affect butterfly reproduction. Rainfall, and freezing temperatures affect milkweed growth and the survival of migrating adult butterflies.Omar Vidal, director general of WWF-Mexico, said “The monarch’s lifecycle depends on the climatic conditions in the places where they breed. Eggs, larvae and pupae develop more quickly in milder conditions. Temperatures above 95°F can be lethal for larvae, and eggs dry out in hot, arid conditions, causing a drastic decrease in hatch rate.”  
A 273-million base pair draft sequence of the monarch butterfly genome was published in 2011, including a set of 16,866 protein-coding genes. Comparison to the sequence of the silk moth Bombyx mori reveals the Lepidoptera as a relatively fast-evolving order. The monarch genome provides a number of insights into the butterfly's migratory behaviour, including the molecular underpinnings of the circadian clock and juvenile hormone pathway, as well as a suite of microRNAs that are differentially expressed between summer and migratory monarchs.
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