Wildlife - The Monarch: A Fight for Survival ~ Pawprints and Purrs

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The Monarch

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In common with many other insects, adult butterflies have antennae, compound eyes, six pair of legs, a hard exoskeleton, and a body that is divided into three parts: the head, thorax, and the abdomen. Uniquely, a butterfly's outer body is covered by tiny sensory hairs and the wings are covered by scales.

The head carries many sensory apparati for the butterfly. A butterfly's compound eye enables it to be aware of its immediate surroundings through a large angle. The pair of eyes are clubbed in most butterflies. On the underside of the head is the paired proboscis, which is used to suck nectar from flowers.

The thorax consists of three segments with a pair of legs attached to each segment. The front pair of legs are non-functional and reduced in length in some families of butterflies. The thorax also contains the flight muscles, which are attached to the base of the wings. Internally, the thorax houses the large muscles that control the wings and legs.

The abdomen contains the bulk of the digestive, excretory, and reproductive organs. At the end of the abdomen are the sexual apparati, which contain many characteristics used by taxonomists as an aid in identifying species.

The life cycle of a butterfly includes four stages: egg, caterpillar or larva, pupa, and adult. The pupa stage is when butterflies undergo a complete metamorphosis. The time needed to complete the metamorphosis varies in each species.

The reproduction process of the butterfly begins with two adults courting and then mating. After mating is complete, the female searches for a location to lay her eggs. It is essential that she finds a place where an appropriate food plant for her larvae is available. Females lay their eggs singly or in groups directly on the underside of the leaf or on the stem of the food plant. Butterfly eggs vary in color, but most intend to be white, green, or yellow, and then change color as the larva develops inside.

When the eggs hatch, the caterpillar enters the first instar (stage of development). Most butterflies experience five instars over a period of three to six weeks. Each time the caterpillar grows bigger, it sheds its skin in a process called molting. After the fifth molting, the caterpillar is usually full grown. It then stops eating and searches for an acceptable place to pupate. It either spins a silken web to fasten the pupa on a firm base or a silken girdle to support the pupa from a stem or a twig. About ten days later, or the next spring for those that hibernate in the winter as pupae, the adult emerges, starting the cycle over.

Migration is found in over 200 butterfly species. Many migrate due to changing seasonal conditions, moving for example to areas that are experiencing a new flush of growth or to areas that are cooler and more moist. Other reasons for migration may include temporary overpopulation and the search for new larval host plants. One of the most well known migrants is the Monarch (Danaus plexippus).

The Monarch Butterfly, because of its size, beauty and spectacular annual migrations is well known to most citizens of the world. Most people do not realize, however, that the Monarch's overwintering roosts in Mexico of are in a precarious state. The destruction of Oyamel fir forests could deal a devastating blow to the survival of this well known and beloved butterfly.

Like birds, the Monarch follows a pattern of seasonal migration. There are two distinct populations in North America, those that breed in the East and those that breed in the West. Each autumn millions leave their breeding grounds and fly to overwintering sites. The Eastern population overwinters in the volcanic mountains of eastern Michoacan in central Mexico. The Western breeders spend their winters along the California coast. Similar migratory behavior has been observed in Costa Rican and Australian populations. About 90 percent of the world's monarchs live east of the Rocky Mountains. Each fall, they migrate 2,000 miles from as far away as southern Canada to their winter quarters in the Oyamel forests.

Entomologists divide the migrating populations of Monarch Butterflies into two groups, one west of the continental divide which is considered too high for the butterflies to fly over, and all the territory eastward including Florida. The eastern and western migrating Monarch undergoes a chemical change delaying sexual maturity, allowing the butterflies to wait out the winter in large colonies south of the freeze line which have been found in Mexico and California. They only mate when they return north, living as long as nine months in the process.

The male Monarch Butterfly may be easily distinguished from the female by noting the two highly visable black spots on the insect's hind wings and the thinner black webbing within the wings. The female's webbing is thicker and she has no identifying wing spot as the male does.

In early spring, the mating period occurs just prior to migration from the overwintering sites. The courtship is fairly simple and less dependent on chemical pheromones in comparison with other species in its genus. Courtship is composed of two distinct stages, the aerial phase and the ground phase. During the aerial phase, the male pursues, nudges, and eventually takes down the female. Copulation occurs during the ground phase and involves the transfer of a spermatophore from the male to female. Along with sperm, the spermatophore is thought to provide the female with energy resources that aid her in carrying out reproduction and remigration.

By early April, the butterflies migrate 800 miles back to the southern U.S., where each female lays about 400 eggs on milkweeds. Many species of milkweed occurring in parts of the United States and Mexico are known to be poisonous to cattle, and the Monarch is considered beneficial because it helps reduce the abundance of these plants. Overwintering sites are of interest to tourists. The egg and larval period is temperature dependent and lasts about 2 weeks. The larva feed on a wide range of milkweeds of the genus Asclepias. From these plants they acquire and store cardiac glycosides, secondary plant compounds that protect them from predation. The adults of the species forage for flower nectar. The eggs turn into caterpillars, the caterpillars into jade-colored chrysalids. After 9 to 15 days, an adult butterfly emerges. After they hatch, this spring generation travels north to Canada. Over the summer, two or three more generations are produced, and the great-grandchildren of those that flew north go back to Mexico to repeat the cycle.

The annual Monarch migration is considered a "threatened phenomena" by the International Union for Conservation of Nature and Natural Resources. Steps have been taken by both the United States and Mexican governments along with numerous private individuals and organizations to protect the overwintering sites of these butterflies.

The Monarch is a valuable model for the study of a variety of biological subjects including host-plant exploitation, migration, overwintering, and chemical defense against natural enemies. Study of this species also provides valuable information that can be applied to the work being done in habitat and species conservation. With its range encompassing all three countries of North America, it has been proposed that the Monarch provides an excellent opportunity for international cooperation in these areas.

Resources:
The Natural History of North America, Edward Ricciuti

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