Common Name: Monarch Butterfly
(Information for this species page was gathered in part by Ray Zerjav for Biology 220W in Spring 2011 at Penn State New Kensington)
The monarch butterfly (Danaus plexippus) is quite possibly the most widely recognized and widely admired insect in the United States. Its large, distinctive black and orange wings, its ability to migrate over thousands of miles to its over-wintering sites, and its life cycle that is intertwined with the ecology and distribution of the milkweed plant all make this species endlessly fascinating to a broad segment of our population.
Image credit - Kenneth Dwain Harrelson, Wikimedia Commons
The monarch’s spread wings are three and a half to four inches across. The wing membranes are orange and its venations are black. There is also a black border on the edges of the wings that is highlighted by white and orange spots. The male monarch is slightly larger than the female and is also typically more brightly colored. The male also has scent patches on its hind wings which are used to produce pheromones. The smaller, less intensely colored female has a greater amount of black pigmentation in its wing veins and borders. Monarchs that are participants in the long migration of the species are usually more deeply orange in color than the “local” individuals. Sometimes this deep orange coloration becomes so intense that it actually looks more red than orange.
Monarchs, like all insects, have six legs (three pair) that are attached to the middle body segment (the thorax). The first pair of legs in monarchs, though, is greatly reduced in size and are held tight against the thorax. These legs have abundant sensory structures and are used by the butterfly to “taste” potential plants to determine their suitability as nectar (“food”) sources. The remaining two pairs of legs are longer and are used for locomotion.
Life Cycle, Mating and Reproduction
The local cycle begins with the adult butterflies emerging from their cocoons (their “chrysalises”). These adult may live for two to five weeks depending primarily on temperature and other weather conditions and also on the availability of their food supply (flower nectar). Females release pheromones which attract males. Females that have not mated release more pheromones than previously mated females and, thus, attract more males. Males fly after the females and force them to the ground to mate. Only about one third of these mating attempts, though, actually result in the transfer of the male’s packet of sperm (the “spermatophore”). Further, males may hold the female in place on the ground for several hours in order to delay transferring their spermatophores until late in the afternoon or early in the evening so that another male of this obligatorily diurnally active species will not be able to immediately mate with that female.
Females will, though, eventually mate with many males, and both the females and their fertilized eggs and larvae benefit nutritionally from the proteins in the accumulated spermatophores. Females that have had the weeks of their adult life span in which to feed and grow are larger than newly emerged females. These older females produce larger eggs which, in turn, hatch into larger and more successful larvae (caterpillars). There is, then, a distinct female strategy to delay copulation (or at least fertilization) until their full growth has been attained.
Relationship with Milkweed
While avoided by vertebrates, the eggs and the larvae are under intense predation pressure from non-vertebrate species. More than ninety percent of the eggs and caterpillars will fail to survive. Eggs are eaten by ants, earwigs and snails, and larvae are eaten by beetles and other insects (like paper wasps) or killed by parasitoid wasps, bacteria, or fungi.
The end stage caterpillar then forms a cocoon (“chrysalis”) within which the tissues and organs of the larvae dissolve and are reformed into the structures of the butterfly. This metamorphosis takes between nine and fifteen days. The emergence of the butterfly from the chrysalis then starts the cycle all over again.
The two migrating populations of monarchs in North America are separated by the Rocky Mountains. The larger area east of the Rockies supports a much larger population of monarchs. All of these butterflies overwinter in the coniferous forests in the mountains of the Mexican states of Michoacán and Mexico. For the monarchs that reach the northeast states of the United States and the southeast provinces of Canada, this migration to and from this very specific overwintering site in Mexico covers several thousand miles. The monarchs that live in the smaller area west of the Rockies, on the other hand, overwinter in coastal sites in Southern and Central California. Their migratory route only measures hundreds of miles at the most. In both overwintering sites, however, the numbers of monarchs covering the trees and shrubs while waiting out the winter months in their diapause states can be truly staggering!
Following the eastern population through their cycle blends together the local and the migratory aspects of the monarch’s life cycle. Between February and March the monarchs who have spent possibly four or five months in their diapause state, re-awaken, mate, and then begin their flight north. They fly as far north as Texas and Oklahoma and out across the southern states. With luck, they have timed their arrival in these areas with the emergence of the new, Spring crop of milkweed. The overwintering migrants then lay their eggs on the milkweed and die. The next generation then undergoes a local life cycle and the adult butterflies mate on emergence and then continue their fight northward in late March and early April. This cohort of adults then gets further north into the Midwest and mid-Atlantic states. This cohort again has ideally timed their northernmost arrival to the emergence of the new crop of milkweed. This first, post-migrant generation then lays their eggs on the milkweed and dies. The second post-migrant generation then undergoes a local life cycle sequence and the emerging adults in June or July head into the northern most states and southern Canada. Again, they lay their eggs near the end of their brief lives and die. The next generation (the third, post-migrant generation) can have two different types of individuals. One type continues on its collective northern flight while the other type turns to the south and gets a head start on the Fall return migration. The northward flying cohort lays its eggs on the northern edge of the milkweed plants while the southward flying cohort lays its eggs on the southern mass of milkweed. Out of these eggs are hatched the larvae that metamorphose into the adults that will be the long-lived migratory life forms that will then attempt to fly all the way back to the coniferous forests in the mountains of Mexico.
The migrating monarchs stop at nectar sites to drink and re-fuel. They follow a variety of cues to stay on their course including polarized light patterns, UV light patterns and the Earth’s geomagnetic fields. They also utilize weather fronts and prevailing winds to give them a flight boost and save a great deal of wear and tear on their delicate wings.
The monarch butterfly is the state insect of seven U.S states. In 1990 it was proposed to be the national insect of the United States, but the legislation did not make it all the way through Congress. This is an iconic species of our North American ecosystems, and is in need of help to make it through the Twenty-first Century.
| The Pennsylvania State University ©2002
This page was last updated on
July 31, 2014