Monarch Butterfly: Life Cycle, Migration, and Conservation

Introduction: The Complex Life of the Monarch Butterfly

Many people think butterflies live in a carefree environment, but they are mistaken. Butterflies seem so peaceful as they visit flowers, yet they are bound by social conventions and instincts of their own. Although their lives appear simple, they are in fact quite demanding (Farrand 1990). The beauty of the Monarch delights most butterfly enthusiasts, but the life of a Monarch butterfly is quite complicated as it navigates the instincts it is bound by. A butterfly's life depends on finding enough food, locating a safe place to lay its eggs, fulfilling the intricate demands of courtship, finding the right spot for transformation from caterpillar to butterfly, fending off predators, and completing migration. One mistake can end the life of a butterfly.

Butterflies are considered invertebrates because they lack a backbone and may be soft- or hard-bodied, with signs of segmentation (Feltwell 1986). Butterflies belong to the class Insecta, found within the phylum Arthropoda. According to Feltwell, all arthropods share six basic characteristics:

Classification and Physical Characteristics

They are wrapped in a tough exoskeleton made of chitin (Feltwell 1986). They are segmented. They have legs or appendages arising from separate segments. They have a haemocoel located between the exoskeleton and the internal organs. They have a dorsal contractile heart and a ventral nervous system (1986).

The butterfly belongs to the order Lepidoptera, a Greek word meaning that the wings are covered in scales. Butterflies have compound eyes on either side of the head, made up of thousands of lensed units called ommatidia. They are not able to see fine detail, but their eyes allow them to detect approaching predators.

The Monarch is one of many varieties of butterfly and can be considered the king of the insect world. Although small, Monarchs accomplish phenomenal things. Danaus plexippus is the scientific name for the Monarch butterfly (Inspecta World 2002). Butterflies come from one of the largest orders of insects (O'Toole 1986). They have overlapping scales on their wings; the fine dust that rubs off when a butterfly is handled consists of these scales. The hair-like structures visible on their legs are also scales. Monarchs can be described as having "large orange-brown wings with white spots, caterpillars with two pairs of black horns at either end of the body. Caterpillars and adults store poisons from the food plant. There are two species in Europe, the Monarch (Danaus plexippus) and the Plain Tiger (Danaus chrysippus)" (Feltwell 1986). However, not all Monarchs are poisonous, because not all milkweed plants produce cardiac glycosides (Inspecta World 2002).

Butterflies have knobs on the tips of their antennae (Encarta 2002). There are approximately 18,500 known species of butterfly. The butterfly's mouth is located between its two eyes. The proboscis, or tongue, is normally coiled beneath its face, but the butterfly can uncoil it and insert it into flowers and other food sources.

The thorax — the middle section of the body — is the thickest part of the butterfly. It contains powerful flight muscles and bears the legs and wings (Encarta 2002). Each of the butterfly's feet bears a pair of claws for clinging to flowers and other surfaces. The wings are located on the sides of the body and are very thin; the scales on the wings give the butterfly its color. The abdomen is tube-like in shape and consists of ten segments. The heart, respiratory system, digestive system, and reproductive organs are all housed in the abdomen. The heart runs the full length of the abdomen. Oxygen enters through six openings in the abdomen called spiracles, which connect to a network of tubes called tracheae. In females, the abdomen is larger because the reproductive organs are located there (Encarta 2002).

The Monarch's bright colors serve an important purpose. The vivid patterns allow other butterflies and observers to identify the species. The Monarch's bold patterns and colors signal that the caterpillar has fed on poisonous milkweed and that the butterfly itself is toxic (Farrand 1990).

The Life Cycle of a Monarch Butterfly

The scientific classification of the Monarch butterfly is as follows: Kingdom Animalia; Class Insecta (insects); Order Lepidoptera (butterflies); Family Danaidae (milkweed butterfly family); Genus Danaus; Species plexippus.

Butterflies lay their eggs on the underside of individual milkweed leaves. It takes approximately two weeks for the eggs to hatch into caterpillars. As this period progresses, the eggs change color from yellow to light gray. Then the caterpillar begins to push its head out of the eggshell.

The newborn caterpillar is only about 2 millimeters long, but it has a voracious appetite. The first thing it does is eat the eggshell. It then consumes milkweed continuously, eating day and night, stopping only briefly to rest between meals. By the time the caterpillar is ready to transform, it is approximately 2,700 times larger than when it was born — yet it is still only about two inches long.

The caterpillar has spiracles, a series of ring-like openings that provide respiration. It has six true legs and five large prolegs, which are used to grip leaves and other surfaces as it moves. Both ends of the body bear a pair of fleshy filaments, though their precise function is not yet known.

When the caterpillar is ready to transform, it grows restless and searches for a safe location. Once a site is chosen, "it uses a special gland in its mouth to weave a small silk button underneath a twig or leaf. It attaches its tail end to the lump" (Inspecta 2002). It then forms a "J" shape as it hangs upside down. Slowly the caterpillar begins to move until its skin splits open. The caterpillar may wriggle for up to five hours to shed the old skin. Once the old skin is gone, the caterpillar resembles a green water droplet. This is called the pupal stage.

The outer layer then hardens into an emerald case dotted with golden spots — a structure known as the chrysalis (Inspecta 2002). Approximately two weeks later, the outline of a butterfly becomes visible through the chrysalis wall. When the butterfly emerges, its wings are wrinkled and wet, and the abdomen is large. The newly emerged butterfly clings to the chrysalis casing while fluid is pumped into the wings, expanding them. Once the wings are dry and the abdomen has reduced in size, the Monarch is able to fly.

Male and female Monarchs can be distinguished by their scent glands. The male's glands appear as a spot of dark scales on the hind wings, while the female has broader black vein lines. The Monarch gathers energy by visiting flowers as it travels, building reserves in preparation for its autumn migration.