Butterflies

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Butterflies

Evolution

Development and life cycle

Vulnerability and defense

Conservation

Resources

Butterflies are insects in the order Lepidoptera, which includes the moths. Butterflies at rest fold their wings vertically over their head, whereas moths hold their wings horizontally. Most butterflies are active during daylight, while moths are mostly nocturnal. Butterflies undergo complete metamorphosis, that is, their egg hatches to a larva (or caterpillar), which pupates in a chrysalis, from which emerges the adult butterfly (or imago).

Evolution

Butterflies probably first evolved about 150 million years ago, appearing at about the same time as the flowering (or angiosperm) plants. Of the 220, 000 species of Lepidoptera, about 45, 000 are butterflies, which probably evolved from moths. Butterflies are found throughout the world, except in Antarctica, and are especially numerous in the tropics. They fall into eight families: Papilionidae (swallowtail butterflies), Pieridae (whites), Danaidae (milkweeds), Satyridae (browns), Morphidae (morphos), Nymphalidae (nymphalids), Lycaenidae (blues), and Hesperidae (skippers).

Development and life cycle

The adult life of butterflies is only one week for some species, but up to 10 months for others. Adult

butterflies spend much of their time courting, mating, and (for females) laying eggs. The females lay eggs on the specific food plant upon which the caterpillar must feed. Eggs are typically laid on leaves, flowers, or stems, but sometimes on treebark or even stones, leaving the newly hatched caterpillars to find their necessary food plant. A few species of butterfly lay their eggs while in flight, the eggs attaching to food plants as they fall.

The egg

By the time most female butterflies emerge from the chrysalis, their eggs are fully mature, permitting immediate fertilization. Minute pores in the egg allow entry of the male sperm for fertilization. Eggs may be laid singly, in rows, in clusters, or in rings around a plant stem. The eggs are highly vulnerable to predators and parasites. Females lay up to 600 eggs at a time. A sticky substance produced during egg laying glues them to the host plant. Most hatch within a few days, although those of some species remain dormant over winter and hatch in the spring.

The caterpillar

The larval stage of butterflies, known as a caterpillar, emerges from the egg fully formed. Caterpillars see by means of groups of tiny eyes on each side of their head. The body of many species is protected by bristly hairs. Caterpillars have as many as eight pairs of appendages. Three pairs are true legs, and become the legs of the adult butterfly. These are located on the thorax behind the head. Another four pairs of appendages occur on the abdomen, and are known as false, or prolegs. There is also a single pair of claspers at the tip of the abdomen, which grip tightly to the food plant. Prolegs and claspers also carry tiny hooks, which catch a silk thread spun by the larva as it moves about its food plant and help keep it from falling. The silk thread is produced as a thick liquid, excreted through glands near the mouth, which is then spun into a thread by a spinneret located behind the jaws.

Caterpillars spend most of their time eating. Their large jaws (the mandibles) move sideways while shredding their plant diet. Caterpillars munch their way through life: first eating their egg shell, from which important nutrients are retrieved, and then continuously consuming its food plants. A very small number of butterfly species have caterpillars that are predators that feed on other insects. Caterpillars typically eat more than twice their own weight each day, pausing only to shed their old skin, in order to grow larger. Skin shedding (molting, or ecdysis) occurs at least four times before the caterpillar is fully grown. Growth rate depends on temperature; it is faster in warm weather, and slower when cool. Caterpillars seek a protected place in which to pupate, forming a protective shell and becoming dormant. Some caterpillars travel rather long distances (330 feet or 100 m) when seeking a sheltered place to pupate.

The chrysalis

The protective case surrounding the pupating caterpillar can take many shapes. It is usually brownish green in color, and may be speckled to aid in camouflage. With few exceptions, butterfly larvae pupate above ground, usually attached to a leaf or stem by a silken thread. Inside the chrysalis the pupated caterpillar gradually transforms into a butterfly. The process takes two weeks for some species, and as long as several years for others. When fully developed, the butterfly inside the chrysalis swallows air, inflating its body and splitting the pupal skin. An adult butterfly struggles out shortly after dawn, its moist wings hanging limply from its thorax. Crawling to a place where it can hang by its legs, the newly emerged butterfly pumps up its wings by swallowing air, increasing its internal pressure and forcing blood through tiny veins in the wings. Within several hours the expanded wings dry and harden. At about the same time the butterfly excretes waste products accumulated during pupation. The butterfly is now free to begin the adult part of its life cycle.

The adult (or imago)

Butterflies, like all insects, have an external skeleton (or exoskeleton) to which muscles are attached. The exoskeleton provides the butterfly’s body with support and reduces water loss through evaporation. The respiratory system does not have a pumping mechanism. The sides of the thorax and abdomen have tiny pores (or spiracles) through which air enters and leaves the body via tubes (tracheae). The insect’s blood (hemo-lymph) is pumped as it passes through a long, thin heart, and bathes the organs inside the body cavity.

Two large compound eyes, made up of hundreds of tiny units (ommatidia), cover much of the butterfly’s head and allow a wide field of vision, including partially backwards. However, the eyes cannot distinguish much detail or determine distance, although they can readily identify color and movement, both of which are vital for survival. Colors aid in the identification of flowers, larval food plants, and the opposite sex of the species, while detecting movement may save butterflies from attacks by their predators.

The long, coiled proboscis (tongue) of the butterfly is projected into the center of flowers while searching for nectar (a liquid, sugar-rich food). Above the eyes and on either side of the head are two antennae covered with microscopic sense organs. The antennae are often incorrectly called “feelers,” but “smellers” would be more accurate because it is through these organs that the butterfly sniffs out its favorite foods and potential mate. The antennae can detect pheromones, which are specific chemical signals released by the opposite sex that are detectable over a great distance.

The thorax of a butterfly is divided into three segments, each having one pair of legs. Each leg ends in a claw, enabling the butterfly to hold on while feeding and egg laying. Sensory receptors on the leg just above the claw detect the chemical makeup of appropriate food plants.

The wings are the most spectacular part of the butterfly, and are extremely large compared to its body. The wings attach at the two rear segments of the thorax. Tough veins provide a framework which supports the wing membrane, covered by millions of microscopic scales arranged in rows like shingles on a roof. It is these scales (about 99,000 per square inch [15,000 per sq cm]) which form the color and pattern of the wing. Brown, orange, and black tones are due to the presence of chemical pigments. However, the brilliant, iridescent blues and greens are parts of the light-spectrum separated by tiny facets on the surface of the scales, creating colors in the fashion of a prism. The wing colors provide camouflage against predators and aid in identifying mates of the correct species. Scales also cover the segmented abdomen, in which the digestive and reproductive organs are located.

Reproduction

Reproduction in butterflies begins with courtship, during which the male vigorously flaps its wings, releasing a dust of microscopic scales carrying pheromones above the female’s antennae. These male pheromones act as a sexual stimulant to the female. Some males release additional pheromones from “hair pencils” under the abdomen. Female butterflies that are ready to mate dispense with courtship. Some species, however, perform complicated courtship maneuvers, probably to find a mate strong enough to endure the rigorous rituals, thereby increasing the chance of producing healthy offspring. Males usually must wait one or two days after emerging from the chrysalis before they can mate, but then they may mate many times. Females can mate immediately after emerging, some species mating several times. However, it is the last male to mate that fertilizes the eggs. Females of some species mate once only.

Migration

Most butterfly species live and die within a narrow geographic range, because their brief life span allows little time for wandering. However, the monarch butterfly migrates thousands of miles from southern Canada and northern parts of the United States to Mexico and southern coastal California. The round trip takes two or three generations to complete, because of the short individual life span. Each butterfly must find its own way, having no living guide, yet the species follows the same broad migratory route year after year, century after century. Monarchs and some other butterflies migrate in massive numbers, forming clouds of color sometimes a mile wide and a mile long; millions perish during the journey.

Vulnerability and defense

The survival of butterflies is influenced by many factors, and their populations may increase or decrease quite rapidly, and from year to year. Brilliantly colored butterflies are often toxic when eaten, so predators learn to leave them alone. Occasionally, a nonpoisonous species evolves that mimics the appearance of a poisonous species, thus being less susceptible to predators. Dark colors, displayed by butterflies in cooler climates (such as alpine or arctic tundra), readily absorb sunlight so the cold-blooded insect can warm up more quickly.

Some butterflies are well camouflaged and almost undetectable among the flowers of their particular food plant. A variety of spots resembling eyes on the outer edges of the wings of certain species may startle or distract predators, drawing the point of attack of a swooping bird away from the butterfly’s soft body. Some butterflies do a 180-degree turn before landing, so the eye-spots on the wings are positioned where a predator may think the head should be. Butterflies surviving bird attacks can often fly well even with a piece of wing missing.

The underpart of the wing is usually duller than the surface, so that when the butterfly assumes its resting position with wings folded above its head, it is cryptically colored, blending into the background on which it lands. Some butterflies use the club on the end of the antennae to knock small predators off their body.

The hairy spines on many caterpillars deter predators, and their colors often blend in with the leaves of their food plant. Some pupae develop hornlike appendages on the head and rear of the chrysalis, which appear to point menacingly at predators. In other cases, a chrysalis hanging from a twig may look like a dead leaf.

Conservation

Habitat loss to agriculture, deforestation, urbanization, draining of wetlands, and other changes in land use is the foremost threat to butterfly populations. Although pollution, pesticides, and specimen collection pose serious threats to some species, none is as damaging as habitat loss. The short life span of butterflies usually makes it impossible for displaced populations to find another appropriate habitat. Although no species is known to have been made extinct through human actions, some subspecies have been rendered extinct, and some rare species are endangered. Protection of habitat is the most effective way to prevent major reductions in populations and endangerment of butterflies, and of other wild animals and plants.

KEY TERMS

Chrysalis— A soft casing, shell, or cocoon protecting the dormant pupa of insects during metamorphosis.

Metamorphosis— A complete change of form, structure, or function in the process of development, shown by insects.

Pheromones— Chemical substances, secreted by most animals, that stimulate a response from others of that species (e.g., sex hormones for mating) or other species (e.g., a predator sensing chemicals produced by its prey’s fear).

Pupa— An insect in the nonfeeding stage during which the larva develops into the adult.