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description [edit]

The monarch butterfly or simply monarch (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae.[4] Other common names depending on region include milkweed, common tiger, wanderer, and black veined brown.[5] It may be the most familiar North American butterfly, and is considered an iconic pollinator species.[6] Its wings feature an easily recognizable black, orange, and white pattern, with a wingspan of 8.9–10.2 cm (​3 1⁄2–4 in)[7] A Müllerian mimic, the viceroy butterfly is similar in color and pattern, but is markedly smaller and has an extra black stripe across each hindwing. The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the northern and central United States and southern Canada to Florida and Mexico.[4] During the fall migration, monarchs cover thousands of miles, with a corresponding multi-generational return north. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in southern California but has been found in overwintering Mexican sites as well.[8][9] Monarchs have been bred on the International Space Station.[10]

Habitat[edit]

Commonly and easily mistaken for the similar viceroy butterfly – the two species are Müllerian mimics, the monarch's wingspan ranges from 8.9 to 10.2 centimetres (3.5–4.0 in).[7] The uppersides of the wings are tawny orange, the veins and margins are black, and there are two series of small white spots in the margins. Monarch forewings also have a few orange spots near their tips. Wing undersides are similar, but the tips of forewings and hindwings are yellow brown instead of tawny orange and the white spots are larger.[24] The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants.[25] Wings size and shape differ between migratory and non-migratory monarchs. Monarchs from eastern North America have larger and more angular forewings than those in the western population.[10]

Monarch flight has been described as "slow and sailing".[26] Monarch flight speed has been estimated by a number of researchers. One scientist examined all prior estimates and concluded their flight speed is approximately 9 km/h or 5.5 mph.[27] For comparison, the average human jogs at a rate of 9.7–12.9 km/h (6–8 mph).

Adults are sexually dimorphic. Males are slightly larger than females[10][24] and have a black patch or spot of androconial scales on each hindwing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs). The male's black wing veins are lighter and narrower than those of females.[28]

One variation, the "white monarch", observed in Australia, New Zealand, Indonesia and the United States, is called nivosus by lepidopterists. It is grayish white in all areas of its wings that are normally orange and is only about 1% or less of all monarchs, but populations as high as 10% exist on Oahu in Hawaii.[17]

The monarch has six legs like all insects, but uses only its middle legs and hindlegs as the forelegs are vestigial, as in all Nymphalidae, and held against its body.[29]

taxonomy[edit]

The monarch butterfly or simply monarch (Danaus plexippus) is a milkweed butterfly (subfamily Danainae) in the family Nymphalidae.[4] Other common names depending on region include milkweed, common tiger, wanderer, and black veined brown.[5] It may be the most familiar North American butterfly, and is considered an iconic pollinator species.[6] Its wings feature an easily recognizable black, orange, and white pattern, with a wingspan of 8.9–10.2 cm (​3 1⁄2–4 in)[7] A Müllerian mimic, the viceroy butterfly is similar in color and pattern, but is markedly smaller and has an extra black stripe across each hindwing.

 

The eastern North American monarch population is notable for its annual southward late-summer/autumn migration from the northern and central United States and southern Canada to Florida and Mexico.[4] During the fall migration, monarchs cover thousands of miles, with a corresponding multi-generational return north. The western North American population of monarchs west of the Rocky Mountains often migrates to sites in southern California but has been found in overwintering Mexican sites as well.[8][9] Monarchs have been bred on the International Space Station.[10]

life cycle[edit]

Commonly and easily mistaken for the similar viceroy butterfly – the two species are Müllerian mimics, the monarch's wingspan ranges from 8.9 to 10.2 centimetres (3.5–4.0 in).[7] The uppersides of the wings are tawny orange, the veins and margins are black, and there are two series of small white spots in the margins. Monarch forewings also have a few orange spots near their tips. Wing undersides are similar, but the tips of forewings and hindwings are yellow brown instead of tawny orange and the white spots are larger.[24] The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants.[25] Wings size and shape differ between migratory and non-migratory monarchs. Monarchs from eastern North America have larger and more angular forewings than those in the western population.[10]

Monarch flight has been described as "slow and sailing".[26] Monarch flight speed has been estimated by a number of researchers. One scientist examined all prior estimates and concluded their flight speed is approximately 9 km/h or 5.5 mph.[27] For comparison, the average human jogs at a rate of 9.7–12.9 km/h (6–8 mph).

Adults are sexually dimorphic. Males are slightly larger than females[10][24] and have a black patch or spot of androconial scales on each hindwing (in some butterflies, these patches disperse pheromones, but are not known to do so in monarchs). The male's black wing veins are lighter and narrower than those of females.[28]

One variation, the "white monarch", observed in Australia, New Zealand, Indonesia and the United States, is called nivosus by lepidopterists. It is grayish white in all areas of its wings that are normally orange and is only about 1% or less of all monarchs, but populations as high as 10% exist on Oahu in Hawaii.[17]

The monarch has six legs like all insects, but uses only its middle legs and hindlegs as the forelegs are vestigial, as in all Nymphalidae, and held against its body.[29]

conservation efforts [edit]

Many of the priority projects that the national strategy identifies will focus on the I-35 corridor extending for 1,500 miles (2,400 km) from Texas to Minnesota that provides spring and summer breeding habitats in the monarch's key migration corridor.[164]There have been a number of national and local efforts underway to establish pollinator habitat along highways and roadways, although this effort is controversial. Conservationists are lobbying transportation departments and utilities to reduce their use of herbicides and specifically encourage milkweed to grow along roadways and power lines.
 

Reducing roadside mowing and application of herbicides during the butterfly breeding season will encourage milkweed growth.[165] Conservationists lobby agriculture companies to set aside areas that remain unsprayed to allow the butterflies to breed.[132] This practice is controversial because of the high risk of butterfly mortality near roads, as several studies have shown that millions of monarchs and other butterflies are killed by cars every year [139] There is also evidence that monarch larvae living near roads experience physiological stress conditions, as evidenced by elevations in their heart rate.[166]

A 2020 resource from the Cooperative Research Programs of the Transportation Research Board developed products for roadway corridors to provide habitat for monarch butterflies and developed tools for roadside managers to optimize potential habitat for monarch butterflies in their road right-of-ways. [167]

threats [edit]

There is increasing concern related to the ongoing decline of monarchs at their overwintering sites; based on a 2014 twenty-year comparison, the overwintering numbers west of the Rocky Mountains have dropped more than 50 percent since 1997 and the overwintering numbers east of the Rockies have declined by more than 90 percent since 1995.[10]

In February 2015, the U.S. Fish and Wildlife Service provided a statistic showing that nearly a billion monarchs have vanished from the overwintering sites since 1990. At that time, one of the main reasons cited was the herbicides used by farmers and homeowners on milkweed, a plant used as a food source, a home and a nursery by the monarchs.[129] A 2016 study also attributed the last decade's ten-fold decline in overwintering numbers of the eastern monarch population to the loss of breeding habitat, namely the many species of milkweed (Asclepias species) that developing larvae require for food; however, scientists believe there are other factors as well.

 

A number of researchers believe milkweed loss during the breeding season is the cause because declines in milkweed abundance are highly correlated with the adoption of herbicide-tolerant genetically modified corn and soybeans, which now constitute 89% and 94% of these crops, respectively, in the U.S.[122] However, correlative evidence does not prove causation, and other possible causes of the overwintering declines have been proposed. A 2018 study has suggested that the decline in milkweed predates the arrival of GM crops.[130]

disruption and habitat [edit]

String of monarchs wintering at the Pismo State Beach Monarch Preserve, 2015

The range of the western and eastern populations of D. plexippus plexippus expands and contracts depending upon the season. The range differs between breeding areas, migration routes, and winter roosts.[10]:(p18) However, no genetic differences between the western and eastern monarch populations exist;[32] reproductive isolation has not led to subspeciation of these populations, as it has elsewhere within the species' range.[10]:(p19)

In the Americas, the monarch ranges from southern Canada through northern South America.[4] It has also been found in BermudaCook Islands,[33] Hawaii,[34][35] Cuba,[36] and other Caribbean islands[10]:(p18) the SolomonsNew CaledoniaNew Zealand,[37] Papua New Guinea,[38] Australia, the Azores, the Canary IslandsMadeiraGibraltar,[39] the Philippines, and North Africa.[40] It appears in the UK in some years as an accidental migrant.[41]

Overwintering populations of D. plexippus plexippus are found in Mexico, California, along the Gulf Coast, year round in Florida, and in Arizona where the habitat has the specific conditions necessary for their survival.[42][43] On the US East Coast, they have overwintered as far north as Lago Mar, Virginia Beach, Virginia.[44] Their wintering habitat typically provides access to streams, plenty of sunlight (enabling body temperatures that allow flight), and appropriate roosting vegetation, and is relatively free of predators. Overwintering, roosting butterflies have been seen on basswoods, elms, sumacs, locusts, oaks, osage-oranges, mulberries, pecans, willows, cottonwoods, and mesquites.[45] While breeding, monarch habitats can be found in agricultural fields, pasture land, prairie remnants, urban and suburban residential areas, gardens, trees, and roadsides – anywhere where there is access to larval host plants.[46] Habitat restoration is a primary goal in monarch conservation efforts. Habitat requirements change during migration. During the fall migration, butterflies must have access to nectar-producing plants. During the spring migration, butterflies must have access to larval food plants and nectar plants.

 

life cycle [edit]

The monarch butterfly undergoes four stages of complete metamorphosis:

 

eggs [edit]

The eggs are derived from materials ingested as a larva and from the spermatophores received from males during mating.[47] Eggs are laid singly on the underside of a young leaf of a milkweed plant during the spring and summer months.[48] The eggs are cream colored or light green, ovate to conical in shape, and about 1.2×0.9 mm in size. The eggs weigh less than 0.5 mg each and have raised ridges that form longitudinally from the point to apex to the base. Though each egg is ​1⁄1000 the mass of the female, she may lay up to her own mass in eggs. Females lay smaller eggs as they age. Larger females lay larger eggs.[47] The number of eggs laid by a female, who may mate several times, ranges from 290 to 1180.[49] Females lay their eggs on the underside of the milkweed leaves; the offspring's consumption of the milkweed benefits health and helps defend them against predators.[50][51] Eggs take 3 to 8 days to develop and hatch into larva or caterpillars.[10]:(p21) Monarchs will lay eggs along the southern migration route.[52]

 

larvae [edit]

The caterpillar goes through five major, distinct stages of growth and after each one, it molts. Each caterpillar, or instar, that molts is larger than the previous as it eats and stores energy in the form of fat and nutrients to carry it through the nonfeeding pupal stage. Each instar usually lasts about 3 to 5 days, depending on various factors such as temperature and food availability.[4]

The second instar larva develops a characteristic pattern of white, yellow and black transverse bands. It is no longer translucent but is covered in short setae. Pairs of black tentacles begin to grow. One pair grows on the thorax and another pair on the abdomen. Like the first instar, second instar larvae usually eat holes in the middle of the leaf, rather than at the edges. The second instar is usually between 6 mm and 1 cm long.

The third instar larva has more distinct bands and the two pairs of tentacles become longer. Legs on the thorax differentiate into a smaller pair near the head and larger pairs further back. These third-stage caterpillars begin to eat along the leaf edges. The third instar is usually between 1 and 1.5 cm long.

The fourth instar has a different banding pattern. It develops white spots on the prolegs near the back of the caterpillar. It is usually between 1.5 and 2.5 cm long.

The fifth instar larva has a more complex banding pattern and white dots on the prolegs, with front legs that are small and very close to the head. A caterpillar at this stage has an enormous appetite, being able to consume a large milkweed leaf in a day. Its length ranges from 2.5 to 4.5 cm.[4]

 

Size comparison between a monarch caterpillar, a queen caterpillar and a black swallowtail caterpillar

At this stage of development, it is relatively large compared to the earlier instars. The caterpillar completes its growth. At this point, it is 4.5 cm long (large specimens can reach 5 cm) and 7 to 8 mm wide, and weighs about 1.5 grams. This can be compared to the first instar, which was 2 to 6 mm long and 0.5 to 1.5 mm wide. Fifth-instar larvae increase in weight 2000 times from first instars. Fifth-stage instar larva can chew through the petiole or midrib of milkweed leaves and stop the flow of latex. After this, they eat more leaf tissue. Before pupation, larvae must consume milkweed to increase their mass. Larvae stop feeding and search for a pupation site.

 

pupa [edit]

Adult monarch emerges from its chrysalis shell. To prepare for the pupa or chrysalis stage, the caterpillar chooses a safe place for pupation, where it spins a silk pad on a downward-facing horizontal surface. At this point, it turns around and securely latches on with its last pair of hindlegs and hangs upside down, in the form of the letter J. After "J-hanging" for about 12–16 hours, it will suddenly straighten out its body and go into peristalsis some seconds before its skin splits behind its head. It then sheds its skin over a period of a few minutes, revealing a green chrysalis. At first, the chrysalis is long, soft, and somewhat amorphous, but over a few hours it compacts into its distinct shape – an opaque, pale-green chrysalis with small golden dots near the bottom, and a gold-and-black rim around the dorsal side near the top. At first, its exoskeleton is soft and fragile, but it hardens and becomes more durable within about a day. At this point, it is about 2.5 cm (1") long and 10–12 mm (3/8–7/16") wide, weighing about 1.2 grams. At normal summer temperatures, it matures in 8–15 days (usually 11–12 days). During this pupal stage, the adult butterfly forms inside. Within a day or so before emerging is due, the exoskeleton first becomes translucent and the chrysalis more bluish. Finally, within 12 hours or so, it becomes transparent, revealing the black and orange colors of the butterfly inside before it ecloses (emerges).

 

adult [edit]

An adult butterfly emerges after about two weeks as a chrysalis, and hangs upside down for a few hours until its wings are dry. Fluids are pumped into the wings, and they expand, dry, and stiffen. The monarch expands and retracts its wings, and once conditions allow, it then flies and feeds on a variety of nectar plants. During the breeding season adults reach sexual maturity in four or five days. However, the migrating generation does not reach maturity until overwintering is complete.[53] Monarchs typically live for two to five weeks during their breeding season.[10]:(pp22-23) Larvae growing in high densities are smaller, have lower survival, and weigh less as adults compared with those growing in lower densities.[54] Monarch metamorphosis from egg to adult occurs during the warm summer temperatures in as little as 25 days, extending to as many as seven weeks during cool spring conditions. During the development, both larvae and their milkweed hosts are vulnerable to weather extremes, predators, parasites and diseases; commonly fewer than 10% of monarch eggs and caterpillars survive.[10]:(pp21-22) However, this is a natural attrition rate for most butterflies, since they are low on the food chain.

 

Reproduction [edit]

Monarch butterfly mating

Healthy males are more likely to mate than unhealthy ones. Females and males typically mate more than once. Females that mate several times lay more eggs.[55] Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus.[56] Male search and capture strategies may influence copulatory success, and human-induced changes to the habitat can influence monarch mating activity at overwintering sites.[57]

Courtship occurs in two phases. During the aerial phase, a male pursues and often forces a female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes.[58] Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating, though some have more success than others.[59][60] During copulation, a male transfers his spermatophore to a female. Along with sperm, the spermatophore provides a female with nutrition, which aids her in egg laying. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more females' eggs.[61]

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