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A butterfly is an insect of the order Lepidoptera, it belongs to either the Hesperioidea (the skippers) or Papilionoidea (all other butterflies) Superfamilies. Some authors have also suggested the inclusion of the superfamily Hedyloidea, the American butterfly moths. They are notable for their unusual life cycle with a larval caterpillar stage, an inactive pupal stage and a spectacular metamorphosis into a familiar and colourful winged adult form. The diverse patterns formed by their brightly coloured wings and their erratic-yet-graceful flight have made butterfly watching a popular hobby.
The Old English word for butterfly was buttorfleoge apparently because butterflies were thought to steal milk. A similar word occurs in Dutch and German originating from the same belief. This is believed to have led to the evolution of its present name form - butterfly.
An alternative folk etymology, prevalent in Great Britain, is that it originated as a contraction of butter-coloured fly referring to the Brimstone Butterfly Gonepteryx rhamni, often the first butterfly of spring. Another such view is that the word butterfly came from a metathesis of "flutterby".
Butterflies are believed to have evolved from a branch of ancestral forms of moths. This branching is believed to have happened in the Cretaceous Period, 65 million to 135 million years ago.  The oldest known fossil 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 18,000 species of butterflies.
Presently butterflies are classified in two superfamilies, Hesperioidea, consisting of the 'skippers' and Papilionoidea or 'true butterflies'. These are sister taxa, so the butterflies collectively are thought to constitute a true clade. Some taxonomists place them all in superfamily Papilionoidea, distinguishing the skippers from the other butterflies at the series level only. In this system, Papilionoidea consists of the series Hesperiiformes (with one family only, the skipper family Hesperiidae) and the series Papilioniformes (with five families).
The five families of true butterflies usually recognized in the Papilionoidea are:-
Family Papilionidae, the Swallowtails and Birdwings
Family Pieridae, the Whites and Yellows
Family Lycaenidae, the Blues and Coppers, also called the Gossamer-Winged Butterflies
Family Riodinidae, the Metalmark butterflies
Family Nymphalidae, the Brush-footed butterflies
The dichotomous classfication 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.Egg, Larva, known as a caterpillar, Pupa (chrysalis), Adult butterfly (imago) 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 and egg stages and thereby survive winters.
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.
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 mutualistic associations with ants. They communicate with the ants using vibrations that are transmitted through the substrate as well as using chemical signals.
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 upto 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 colours. 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 evolutionary arms race has lead to coevultion in the 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 express molecular markers in 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 subtrate 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.
Adult or Imago
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), but, unlike moths, 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.
Source : http://en.wikipedia.org/wiki/Butterfly