Monarch Butterfly

Danaus plexippus

by Trey Blankinship

 The monarch butterfly is perhaps the most well known of all butterflies.  Butterflies are in the phylum Arthropoda and in the class Insecta.  They are members of the insect order Lepidoptera because they have scales covering their body and wings.  The monarch is in the family Danaidae which includes the milkweed butterflies.  Finally, it belongs to the genus Danaus and to the species plexippus.  It was given the name “monarch” after King William, Prince of Orange, and later King of England.   

A monarch butterfly will lay her eggs on the underside of a milkweed leaf.  In about five to seven days the eggs will hatch and new larvae are born.  When a larva hatches it usually eats some or all of its egg shell for its first meal and during its first day of life will consume its own weight in food.  A larva, commonly known as a caterpillar,  is about two inches long and marked with bands of white, black and yellow.  It has three pairs of true legs which are segmented and located on the thorax.  It also has five pairs of prolegs that are used for gripping and are located on the abdomen.  The larva is an eating machine, feeding on the leaves of plants.  As the larva develops it will periodically molt, or shed its skin.  The time between each molt is called an instar, during which time it may change its appearance drastically.  A larva will go through five instars with its main change being in color pattern and also the legs will move closer to the head.  The larval stage will typically last two to three weeks.             

At this time the caterpillar is ready to begin its pupal stage.  This is really just the final instar during which the caterpillar forms a chrysalis around its body.  It will take the larva twelve to nineteen hours to make this shell.  After it’s completed the larva undergoes a metamorphosis and develops into a butterfly.  It will also develop a reproductive system by the time the process is over.  Depending on temperature, the pupal stage will last one to two weeks.  When it’s time to emerge, the new butterfly will push its wings against the chrysalis and let itself out.           

The development of the monarch butterfly is now completed.  The adult monarch is patterned in brown, orange, and black with an average wingspan of three to four inches.  Like all insects, it is ectothermic, meaning it regulates its body temperature by exchanging heat with its surroundings. To keep its body temperature at a comfortable level, it will usually spread its wings to absorb the warmth of the sun when needed.  This species is also diurnal which means it is active during the daytime rather than at night.  Once the monarch becomes an adult butterfly it will live for about thirty to forty days.           

The monarch butterfly restricts its menu to the milkweed plant.  The milkweed indirectly controls where you will find monarch populations and regulates their density in a given area.  An adult monarch feeds on the nectar made by the plant and uses its proboscis, or sucking tube, to extract the nectar.  When it’s time for the female to lay her eggs, she will lay between four and six hundred eggs on the underside of the leaves.           

The monarch’s original home is in North America, but it is also present on many islands of the South Pacific.  They got to these places after being swept offshore and landing on ships, which carried them to distant islands.  There are two main populations of the monarch butterfly.  One is west of the Rocky Mountains and migrates to California for their overwintering site.  The other, much larger, Eastern population overwinters in central Mexico.  Once they reach their wintering site they form colonies, called roosts, and cling to branches and trunks of trees.  These colonies of butterflies can be in the hundreds of millions.  Finally, they go into a period of inactivity, known as diapause, where they stay from November through early March.  Roosts are also thought to serve a social function where information is shared.           

All insects have predators and the monarch is no exception.  The butterfly is the rarest stage of its life history.  Its main predators are many different species of spiders.  Spiders like to make their webs around the milkweed plants and catch monarchs as they search for food.  Some other predators include praying mantids, hornets, some ants, lizards, dragonflies and even some carnivorous plants.  Bird predation is rare, but it does happen.  Monarch butterflies gain protection from predators by making it known that they contain foul-tasting chemicals.  Their bright colors and elaborate patterns are warning signs to stay away.   The milkweed plant contains toxins, called cardiac glycosides, in its sap that are poisonous to birds.  With a diet of only milkweed, these toxins build up in the tissue of the larva and are passed on to the adult monarch.  Birds and other predators who try to eat the monarch will usually throw up.  Certain tests have shown that some predators have become sick and others have died after consuming a monarch.  Cardiac glycosides are poisonous to vertebrates and after trying this distasteful monarch, predators learn to avoid this species in the future.  The viceroy butterfly also gains protection by mimicking the appearance of the monarch, so that birds won’t eat them either. 

Written spring 2004, as a service learning project for Dr. Gary Coté's Biology 102 class at Radford University.  Copyright Pathways for Radford.