The monarch butterfly, which is sometimes referred to as the milkweed butterfly, common tiger butterfly, wanderer butterfly, and black veined brown butterfly and is known to botanists as the Danaus plexippus, is a milkweed butterfly of the subfamily Danainae in the family Nymphalidae. An iconic pollinator, it is probably the North American butterfly most familiar to Americans. Its numbers in California have dropped precipitously in the last several decades, however, and the western population is moving toward extinction.
The eastern and western populations of North American monarch are notable for their annual southward late-summer/autumn migrations, covering thousands of miles. Eastern monarchs originating in the northern and central United States and southern Canada end up in Florida and Mexico. The lion’s share of the North American population of monarchs west of the Rocky Mountains migrates to sites in southern California, with a somewhat smaller number continuing southward to overwinter in Mexico. The succeeding generation of monarchs return north in the spring.
Easily recognizable by its wings’ black, orange, and white pattern, the monarch has a wingspan of  3 1⁄2 to four inches. The uppersides of the monarch butterfly’s 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. The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants. 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.
Monarch adults exhibit gender dimorphism. Males are slightly larger than females and have a black patch or spot of androconial scales on each hindwing. The male’s black wing veins are lighter and narrower than those of females. Males tend to have larger wings than females, and are heavier than females, on average. Both males and females have similar thorax dimensions, as the wing muscles are contained in the thorax. Female monarchs tend to have thicker wings, which gives them greater tensile strength. This makes female wings less likely to be damaged during migration. Also, females have lower wing loading, the value derived from the ratio of wing size to body weight, than males. Thus, females require less energy to fly.
The monarch has six legs like all insects, but uses only its middle legs and hindlegs. The forelegs are vestigial, as in all other Nymphalidae, and held against its body.
Monarchs, like most butterflies, waft in their flight, and thus appear to be slow. In actuality, they move along at clip of roughly 5.5 miles per hours.
The range of the western and eastern populations of the monarch butterfly expands and contracts depending upon the season. The range differs between breeding areas, migration routes, and winter roosts. However, no genetic differences between the western and eastern monarch populations exist; reproductive isolation has not led to subspeciation of these populations, as it has elsewhere within the species’ range.
Their wintering habitat typically provides access to streams, plenty of sunlight (enabling body temperatures that allow flight), and appropriate roosting vegetation, and relative avoidance of predators. While breeding, monarch habitats can be found anywhere where there is access to larval host plants. 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.
The monarch butterfly undergoes five stages of complete metamorphosis. Eggs are laid singly on the underside of a young leaf of a milkweed plant during the spring and summer months. 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. The number of eggs laid by a female, who may mate several times, ranges from 290 to 1,180. Females lay their eggs on milkweed, which makes their offspring healthier. Eggs take 3 to 8 days to develop and hatch into larva or caterpillars. Monarchs will lay eggs along the southern migration route.
After each stage of growth, the caterpillar molts. The first instar caterpillar that emerges out of the egg is pale green and translucent. The second instar larva develops a characteristic pattern of white, yellow and black transverse bands and grows black tentacles. The third instar larva begins to eat along the leaf edges. The fourth instar develops white spots on the prolegs. 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. 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. When the caterpillar completes its growth, it is one inch to one-and-four fifths inch long and five to eight millimeters wide. Fifth-instar larvae increase in weight 2000 times from first instars and begin to eat more leaf tissue. Before pupation, larva must consume milkweed to increase their mass.
At some point, larva stop feeding and search for a pupation site. The caterpillar attaches itself securely to a horizontal surface, using a silk pad. 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 larva and their milkweed hosts are vulnerable to weather extremes, predators, parasites and diseases; commonly fewer than 10 percent of monarch eggs and caterpillars survive.
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. Monarchs typically live for two to five weeks during their breeding season.
Healthy males are more likely to mate than unhealthy ones. Females and males typically mate more than once. Mating for the overwintering populations occurs in the spring, prior to dispersion. Mating is less dependent on pheromones than other species in its genus.
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. 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.
The name “monarch” may be in honor of King William III of England. The monarch was originally described by Carl Linnaeus in his Systema Naturae of 1758 and placed in the genus Papilio.
The U.S. Fish and Wildlife Service is considering giving monarch butterflies west of the Rockies Endangered Species Act protection, as the number of monarchs surveyed in California is down by more than 90 percent from the early 1980s. A recently completed study, funded by the U.S. Fish and Wildlife Service and published in the scientific journal Biological Conservation, concluded the western monarchs have a 63 percent chance of extinction in 20 years and an 84 percent chance in 50 years if current trends continue. The western population has declined to about 300,000 from 10 million less than four decades ago, scientists say, due to climate change, the destruction of roosting forests in California and Mexico and the increasing use of pesticides that kill milkweed plants. Habitat restoration is a primary goal in monarch conservation efforts.