| Monarch butterfly | |
|---|---|
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| Female | |
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| Male | |
| Binomial name | |
| Danaus plexippus (Linnaeus, 1758) | |
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| Synonyms | |
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 pollinatorspecies.[6] Its wings feature an easily recognizable black, orange, and white pattern, with a wingspan of 8.9–10.2 cm (31⁄2–4 in)[7] 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 were transported to the International Space Station and were bred there.
The name "monarch" may be in honor of King William III of England.[11] The monarch was originally described by Carl Linnaeus in his Systema Naturae of 1758 and placed in the genus Papilio.[12] In 1780, Jan Krzysztof Kluk used the monarch as the type species for a new genus Danaus.
Danaus (Ancient Greek Δαναός), a great-grandson of Zeus, was a mythical king in Egypt or Libya, who founded Argos; Plexippus (Πλήξιππος) was one of the 50 sons of Aegyptus, the twin brother of Danaus. In Homeric Greek, his name means "one who urges on horses", i.e. "rider" or "charioteer". In the 10th edition of Systema Naturae, at the bottom of page 467,[13]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. Linnaeus divided his large genus Papilio, containing all known butterfly species, into what we would now call subgenera. The Danai festivi formed one of the "subgenera", containing colorful species, as opposed to the Danai candidi, containing species with bright white wings. Linnaeus wrote: "Danaorum Candidorum nomina a filiabus Danai Aegypti, Festivorum a filiis mutuatus sunt." (English: "The names of the Danai candidi have been derived from the daughters of Danaus, those of the Danai festivi from the sons of Aegyptus.")
Robert Michael Pyle suggested Danaus is a masculinized version of Danaë (Greek Δανάη), Danaus's great-great-granddaughter, to whom Zeus came as a shower of gold, which seemed to him a more appropriate source for the name of this butterfly.[14]
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Commonly and easily mistaken for the similar viceroy butterfly, 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/hr or 5.5 mph.[27] For comparison, the average human jogs at a rate of 6–8 mph.
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In the Americas, the monarch ranges from southern Canada through northern South America.[4] It has also been found in Bermuda, Cook Islands,[33]Hawaii,[34][35] Cuba,[36] and other Caribbean islands[10]:(p18) the Solomons, New Caledonia, New Zealand,[37] Papua New Guinea,[38] Australia, the Azores, the Canary Islands, Gibraltar,[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 milkweed; the offsprings' 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.[4]
The first instar caterpillar that emerges out of the egg is pale green and translucent. It lacks banding coloration or tentacles. The larvae or caterpillar eats its egg case and begins to feed on milkweed. It is during this stage of growth that the caterpillar begins to sequester cardenolides. The circular motion a caterpillar uses while eating milkweed prevents the flow of latex that could entrap it.
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.
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 fourth instar has a different banding pattern. It develops white spots on the prolegs near the back of the caterpillar.
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.
At this stage of development, it is relatively large compared to the earlier instars. The caterpillar completes its growth. At this point, it is 25 to 45 mm long and 5 to 8 mm wide. 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 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. The caterpillar attaches itself securely to a horizontal surface, using a silk pad. At this point, it latches on with its hindlegs and hangs down. It then molts into an opaque, blue-green chrysalis with small gold dots. At normal summer temperatures, it matures in 8–15 days. The cuticle of the chrysalis becomes transparent and the monarch's characteristic orange-and-black wings become visible. At the end of metamorphosis, the adult emerges from the chrysalis, expands and dries its wings, and flies away. 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)
Pupa[edit]
In the pupa or chrysalis stage, the caterpillar spins a silk pad on to a horizontal substrate. It then hangs from the pad by the last pair of prolegs upside down, resembling the letter "J". It sheds its skin, leaving itself encased in an articulated green exoskeleton. During this pupal stage, the adult butterfly forms inside. The exoskeleton becomes transparent before it ecloses(emerges), and its adult colors can finally be seen.
Adult[edit]
An adult butterfly emerges after about two weeks as a chrysalid, and hangs upside down until its wings are dry. Fluids are pumped into the wings, and they expand 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 to lower densities.[54]
Reproduction[edit]
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]
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.[57] Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating, though some have more success than others.[58][59] 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.[60]
Pictorial life cycle[edit]
Migration[edit]
In North America, monarchs migrate both north and south on an annual basis.[4] The population east of the Rocky Mountainsmigrates to the sanctuaries of the Mariposa Monarca Biosphere Reserve in Mexico and parts of Florida. The western population overwinters in various coastal sites in central and southern California. 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.[69] Captive-raised monarchs appear capable of migrating to overwintering sites in Mexico,[70] though they have a much lower migratory success rate than wild monarchs do.[71] Recent discoveries have located monarch overwintering sites in Arizona.[72]
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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. Conservationists lobby agriculture companies to set aside areas that remain unsprayed to allow the butterflies to breed.[115] 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 [122]
While there are few scientific studies on the subject, the practice of butterfly gardening and creating "Monarch Waystations" is commonly thought to increase the populations of butterflies.[145] Efforts to increase monarch populations by establishing butterfly gardens and waystations require particular attention to the butterfly's food preferences and population cycles, as well to the conditions needed to propagate milkweed. For example, in the Washington, D.C. area and elsewhere in the northeastern United States, monarchs prefer to reproduce on common milkweed (Asclepias syriaca), especially when its foliage is soft and fresh. As monarch reproduction in that area peaks in late summer when milkweed foliage is old and tough, A. syriaca needs to be cut back in June - August to assure that it will be regrowing rapidly when monarch reproduction reaches its peak. In addition, milkweed seed may need a period of cold treatment before it will germinate
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