Butterfly

A butterfly is an insect of the order Lepidoptera. The scope of the term depends on how far the concept of "butterfly" is extended. Currently, most experts include the superfamilies Hedyloidea (the American moth-butterflies), Hesperioidea (the skippers) and Papilionoidea (the so-called 'true' butterflies). This concept of butterflies including the Hedyloidea is an only recently expanded one, but it makes the group a natural clade, the Rhopalocera. Like all Lepidoptera, butterflies are notable for their unusual life cycle with a larval caterpillar stage, an inactive pupal stage and a spectacular metamorphosis into a familiar and colorful winged adult form, and most species being day-flying, they regularly attract attention. The diverse patterns formed by their brightly colored wings and their erratic-yet graceful-flight have made butterfly watching a popular hobby.

Butterflies are nested within the evolutionary tree of moths. Their origins may date back to the Cretaceous Period, ending 65 million years ago. Unfortunately, the fossil record is very limited. The oldest known fossil is an unnamed possible skipper butterfly from the Upper Palaeocene (around 57 million year old) of Fur, Denmark . One of the most beautifully preserved is a Metalmark butterfly (Voltinia dramba) from 25 million year old Dominican amber.

Butterflies are today distributed throughout the world except in the very cold and arid regions. There are an estimated 17,500 species of butterflies (Papilionoidea) out of about 180,000 species of Lepidoptera.

Presently butterflies are classified in three superfamilies, Hedyloidea, consisting of the 'American moth-butterflies', Hesperioidea, consisting of the 'skippers' and Papilionoidea or 'true butterflies'. The last two superfamilies are probably sister taxa, so the butterflies collectively are thought to constitute a natural group or clade.

A study combining morphological and molecular data concluded that Hesperiidae, Papilionidae, Pieridae, Lycaenidae and Riodinidae could all be strongly supported as monophyletic groups, but the monophyletic status of Nymphalidae is uncertain. Lycaenidae and Riodinidae were confirmed as sister taxa, and Papilionidae as the outgroup to the rest of the true butterflies, but the location of Pieridae within the pattern of descent was unclear, with different lines of evidence suggesting different conclusions. The data suggested that the Hedyloidea are indeed more closely related to the butterflies than to other moths.

Some older classifications recognize additional families, for example Danaidae, Heliconiidae, Libytheidae and Satyridae, but modern classifications treat these as subfamilies within the Nymphalidae.

The dichotomous classification of lepidopterans into butterflies and moths is one that is popular but not used in taxonomy. The folk groups of butterflies and moths can be distinguished using several features but there are exceptions to most of these rules.

Unlike many insects, butterflies do not experience a nymph period, but instead go through a pupal stage which lies between the larva and the adult stage (the imago). Butterflies are termed as holometabolous insects, and go through complete metamorphosis.

It is a popular belief that butterflies have very short life spans. However, butterflies in their adult stage can live from a week to nearly a year depending on the species. Many species have long larval life stages while others can remain dormant in their pupal or egg stages and thereby survive winters.

Butterflies may have one or more broods per year. The number of generations per year varies from temperate to tropical regions with tropical regions showing a trend towards multivoltinism.

Butterfly eggs consist of a hard-ridged outer layer of shell, called the chorion. This is lined with a thin coating of wax which prevents the egg from drying out before the larva has had time to fully develop. Each egg contains a number of tiny funnel-shaped openings at one end, called micropyles; the purpose of these holes is to allow sperm to enter and fertilize the egg. Butterfly and moth eggs vary greatly in size between species, but they are all either spherical or ovate.

Butterfly eggs are fixed to a leaf with a special glue which hardens rapidly. As it hardens it contracts deforming the shape of the egg. This glue is easily seen surrounding the base of every egg forming a meniscus. The nature of the glue is unknown, and is a suitable subject for research. The same glue is produced by a pupa to secure the setae of the cremaster. This glue is so hard that the silk pad, to which the setae are glued, cannot be separated.

Eggs are usually laid on plants. Each species of butterfly has its own hostplant range and while some species are restricted to just one species, others use a range of plant species, often members of a common family.

The egg stage lasts a few weeks in most butterflies but eggs laid close to winter especially in temperate regions go through a diapause stage and the hatching may take place only in spring. Other butterflies may lay their eggs in the spring and have them hatch in the summer. These butterflies are usually northernly species (Mourning cloak, Tortiseshells)

Larvae, or caterpillars, are multi-legged eating machines. They consume plant leaves and spend practically all of their time in search of food. Although most caterpillars are herbivorous, a few species such as Spalgis epius and Liphyra brassolis are entomophagous (insect eating). Some larvae, especially those of the Lycaenidae form mutual associations with ants. They communicate with the ants using vibrations that are transmitted through the substrate as well as using chemical signals. The ants provide some degree of protection to these larvae and they in turn gather honeydew secretions.

Caterpillars mature through a series of stages, called instars. Near the end of each instar, the larva undergoes a process called apolysis, in which the cuticle, a mixture of chitin and specialized proteins, is released from the epidermis and the epidermis begins to form a new cuticle beneath. At the end of each instar, the larva moults the old cuticle, and the new cuticle rapidly hardens and pigments. Development of butterfly wing patterns begins by the last larval instar.

Butterfly caterpillars have three pairs of true legs from the thoracic segments and up to 6 pairs of prolegs arising from the abdominal segments. These prolegs have rings of tiny hooks called crochets that help them grip the substrate.

Some caterpillars have the ability to inflate parts of their head to appear snake-like. Many have false eye-spots to enhance this effect. Some caterpillars have special structures called osmeteria which are everted to produce smelly chemicals. These are used in defense.

Host plants often have toxic substances in them and caterpillars are able to sequester these substances and retain them into the adult stage. This helps making them unpalatable to birds and other predators. Such unpalatibility is advertised using bright red, orange, black or white warning colors. The toxic chemicals in plants are often evolved specifically to prevent them from being eaten by insects. Insects in turn develop countermeasures or make use of these toxins for their own survival. This "arms race" has led to the coevolution of insects and their host plants.

Wings or wing pads are not visible on the outside of the larva, but when larvae are dissected, tiny developing wing disks can be found on the second and third thoracic segments, in place of the spiracles that are apparent on abdominal segments. Wing disks develop in association with a trachea that runs along the base of the wing, and are surrounded by a thin peripodial membrane, which is linked to the outer epidermis of the larva by a tiny duct.

Wing disks are very small until the last larval instar, when they increase dramatically in size, are invaded by branching tracheae from the wing base that precede the formation of the wing veins, and begin to develop patterns associated with several landmarks of the wing.

Near pupation, the wings are forced outside the epidermis under pressure from the hemolymph, and although they are initially quite flexible and fragile, by the time the pupa breaks free of the larval cuticle they have adhered tightly to the outer cuticle of the pupa (in obtect pupae). Within hours, the wings form a cuticle so hard and well-joined to the body that pupae can be picked up and handled without damage to the wings.

When the larva is fully grown, hormones such as prothoracicotropic hormone (PTTH) are produced. At this point the larva stops feeding and begins "wandering" in the quest of a suitable pupation site, often the underside of a leaf.

The larva transforms into a pupa (or chrysalis) by anchoring itself to a substrate and moulting for the last time. The chrysalis is usually incapable of movement, although some species can rapidly move the abdominal segments or produce sounds to scare potential predators.

The pupal transformation into a butterfly through metamorphosis has held great appeal to mankind. To transform from the miniature wings visible on the outside of the pupa into large structures usable for flight, the pupal wings undergo rapid mitosis and absorb a great deal of nutrients. If one wing is surgically removed early on, the other three will grow to a larger size. In the pupa, the wing forms a structure that becomes compressed from top to bottom and pleated from proximal to distal ends as it grows, so that it can rapidly be unfolded to its full adult size. Several boundaries seen in the adult color pattern are marked by changes in the expression of particular transcription factors in the early pupa.

The adult, sexually mature, stage of the insect is known as the imago. As Lepidoptera, butterflies have four wings that are covered with tiny scales (see photo. The fore and hindwings are not hooked together, permitting a more graceful flight. An adult butterfly has six legs, but in the nymphalids, the first pair is reduced. After it emerges from its pupal stage, a butterfly cannot fly until the wings are unfolded. A newly-emerged butterfly needs to spend some time inflating its wings with blood and letting them dry, during which time it is extremely vulnerable to predators. Some butterflies wings may take up to 3 hours to dry while others take about 1 hour. Most butterflies and moths will excrete excess dye after hatching. This fluid may be white, red, orange, or in rare cases, blue.

Butterflies are characterized by their scale covered wings. The coloration of butterfly wings is created by minute scales. These scales are pigmented with melanins that give them blacks and browns, but blues, greens, reds and iridescence are usually created not by pigments but the microstructure of the scales. This structural coloration is the result of coherent scattering of light by microstructure of the scales.

Many adult butterflies exhibit polymorphism, showing differences in appearance. These variations include geographic variants and seasonal forms. In addition many species have females in multiple forms, often with mimetic forms. Sexual dimorphism in coloration and appearance is widespread in butterflies. In addition many species show sexual dimorphism in the patterns of ultraviolet reflectivity, while otherwise appearing identical to the unaided human eye. Most of the butterflies have a sex-determination system that is represented as ZW with females being the heterogametic sex (ZW) and males homogametic (ZZ).

Genetic abnormalities such as gynandromorphy also occur from time to time. In addition many butterflies are infected by Wolbachia and infection by the bacteria can lead to the conversion of males into females or the selective killing of males in the egg stage.

Batesian and Mullerian mimicry in butterflies is common. Wing markings called eyespots are present in some species; these may have an automimicry role for some species. In others, the function may be intraspecies communication, such as mate attraction. In several cases, however, the function of butterfly eyespots is not clear, and may be an evolutionary anomaly related to the relative elasticity of the genes that encode the spots.

Many of the tropical butterflies have distinctive seasonal forms. This phenomenon is termed seasonal polyphenism and the seasonal forms of the butterflies are called the dry-season and wet-season forms. How the season affects the genetic expression of patterns is still a subject of research. Experimental modification by ecdysone hormone treatment has demonstrated that it is possible to control the continuum of expression of the different forms. The dry-season forms are usually more cryptic and it has been suggested that the protection offered may be an adaptation. Some also show greater dark colors in the wet-season form which may have thermoregulatory advantages by increasing ability to absorb solar radiation.

Butterflies feed primarily on nectar from flowers. Some also derive nourishment from pollen, tree sap, rotting fruit, dung, and dissolved minerals in wet sand or dirt. Butterflies play an important ecological role as pollinators.

As adults, butterflies consume only liquids and these are sucked by means of their proboscis. They feed on nectar from flowers and also sip water from damp patches. This they do for water, for energy from sugars in nectar and for sodium and other minerals which are vital for their reproduction. Several species of butterflies need more sodium than provided by nectar. They are attracted to sodium in salt and they sometimes land on people, attracted by human sweat. Besides damp patches, some butterflies also visit dung, rotting fruit or carcasses to obtain minerals and nutrients. In many species, this Mud-puddling behavior is restricted to the males and studies have suggested that the nutrients collected are provided as a nuptial gift along with the spermatophore during mating.

Butterflies sense the air for scents, wind and nectar using their antennae. The antennae come in various shapes and colors. The hesperids have a pointed angle or hook to the antennae, while most other families show knobbed antennae. The antennae are richly covered with sensillae. A butterfly's sense of taste is coordinated by chemoreceptors on the tarsi, which work only on contact, and are used to determine whether an egg-laying insect's offspring will be able to feed on a leaf before eggs are laid on it. Many butterflies use chemical signals, pheromones, and specialized scent scales (androconia) and other structures (coremata or 'Hair pencils' in the Danaidae) are developed in some species.

Vision is well developed in butterflies and most species are sensitive to the ultraviolet spectrum. Many species show sexual dimorphism in the patterns of UV reflective patches. Color vision may be widespread but has been demonstrated in only a few species.

Some butterflies have organs of hearing and some species are also known to make stridulatory and clicking sounds.

Many butterflies, such as the Monarch butterfly, are migratory and capable of long distance flights. They migrate during the day and use the sun to orient themselves. They also perceive polarized light and use it for orientation when the sun is hidden.

Many species of butterfly maintain territories and actively chase other species or individuals that may stray into them. Some species will bask or perch on chosen perches. The flight styles of butterflies are often characteristic and some species have courtship flight displays. Basking is an activity which is more common in the cooler hours of the morning. Many species will orient themselves to gather heat from the sun. Some species have evolved dark wingbases to help in gathering more heat and this is especially evident in alpine forms.

Like many other members of the insect world, the lift generated by butterflies is more than what can be accounted for by steady-state, non-transitory aerodynamics. Studies using Vanessa atalanta in a windtunnel show that they use a wide variety of aerodynamic mechanisms to generate force. These include wake capture, vortices at the wing edge, rotational mechanisms and Weis-Fogh 'clap-and-fling' mechanisms. The butterflies were also able to change from one mode to another rapidly. (See also Insect flight)

Many butterflies migrate over long distances. Particularly famous migrations being those of the Monarch butterfly from Mexico to North America, a distance of about 4,000 to 4,800 kilometres (2500-3000 miles). Other well known migratory species include the Painted Lady and several of the Danaine butterflies. Spectacular and large scale migrations associated with the Monsoons are seen in peninsular India. Migrations have been studied in more recent times using wing tags and also using stable hydrogen isotopes.

Butterflies have been shown to navigate using time compensated sun compasses. They can see polarized light and therefore orient even in cloudy conditions. The polarized light in the region close to the ultraviolet spectrum is suggested to be particular important.

It is suggested that most migratory butterflies are those that belong to semi-arid areas where breeding seasons are short. The life-histories of their host plants also influence the strategies of the butterflies.

Butterflies are threatened in their early stages by parasitoids and in all stages by predators, diseases and environmental factors. They protect themselves by a variety of means.

Chemical defenses are widespread and are often based on chemicals of plant origin. In many cases the plants themselves have evolved these toxic substances to reduce attack to them. These defense mechanisms are effective only if they are also well advertised. Many unpalatable butterflies are brightly colored. This has led to unprotected butterflies evolving forms that appear like the unpalatable butterflies. These mimetic forms are usually restricted to the females.

Cryptic coloration is found in many butterflies. Some like the oakleaf butterfly are remarkable imitations of leaves. As caterpillars, many defend themselves by freezing and appearing like sticks or branches. Some papilionid caterpillars resemble bird dropping in their early instars. Some caterpillars have hairs and bristly structures that provide protection while others are gregarious and form dense aggregations. Some species also form associations with ants and gain their protection (See Myrmecophile).

Behavioral defenses include perching and wing positions to avoid being conspicuous. Some female Nymphalid butterflies are known to guard their eggs from parasitoid wasps.

Eyespots and tails are found in many lycaenid butterflies and these divert the attention of predators from the more vital head region. An alternative theory is that these cause ambush predators such as spiders to approach from the wrong end and allow for early visual detection.