Butterflies are winged insects from the lepidopteran superfamily Papilionoidea, characterised by large, often brightly coloured wings that often fold together when at rest, and a conspicuous, fluttering flight. The oldest butterfly fossils have been dated to the Paleocene, about 56 million years ago, though molecular evidence suggests that they likely originated in the Cretaceous.
As holometabolous insects, butterflies undergo complete metamorphosis, meaning that their life cycle comprises four stages. Winged adults lay eggs on plant foliage on which their larvae, known as caterpillars, will feed. The caterpillars grow, sometimes very rapidly, and when fully developed, pupate in a chrysalis. When metamorphosis is complete, the pupal skin splits, the adult insect climbs out, expands its wings to dry, and flies off.
Some butterflies, especially in the tropics, have several generations in a year, while others have a single generation, and a few in cold locations may take several years to pass through their entire life cycle.
Butterflies are often polymorphic, and many species make use of camouflage, mimicry, and aposematism to evade their predators. Phaeism is when a butterfly doesn't fully undergo melanism. Some, like the monarch and the painted lady, migrate over long distances. Many butterflies are attacked by parasites or parasitoids, including wasps, protozoans, flies, and other invertebrates, or are preyed upon by other organisms. Some species are pests because in their larval stages they can damage domestic crops or trees; other species are agents of pollination of some plants. Larvae of a few butterflies (e.g., harvesters) eat harmful insects, and a few are predators of ants, while others live as mutualists in association with ants. Culturally, butterflies are a popular motif in the visual and literary arts. The Smithsonian Institution says "butterflies are certainly [among] the most appealing creatures in nature".
Etymology
The Oxford English Dictionary derives the word straightforwardly from Old English butorflēoge, butter-fly; similar names in Old Dutch and Old High German show that the name is ancient, but modern Dutch and German use different words ( and ) and the common name often varies substantially between otherwise closely related languages. A possible source of the name is the bright yellow male of the brimstone (Gonepteryx rhamni); another is that butterflies were on the wing in meadows during the spring and summer butter season while the grass was growing.Donald A. Ringe, A Linguistic History of English: From Proto-Indo-European to Proto-Germanic (Oxford: Oxford, 2003), 232.
Paleontology
The earliest Lepidoptera fossils date to the Triassic-Jurassic boundary, around 200million years ago. Butterflies evolved from moths, so while the butterflies are monophyletic (forming a single clade), the moths are not. The oldest known butterfly is Protocoeliades kristenseni from the Palaeocene aged Fur Formation of Denmark, approximately 55million years old, which belongs to the family Hesperiidae (skippers). Molecular clock estimates suggest that butterflies originated sometime in the Late Cretaceous, but only significantly diversified during the Cenozoic, with one study suggesting a North American origin for the group. The oldest American butterfly is the Late Eocene Prodryas persephone from the Florissant Fossil Beds, approximately 34million years old.
Taxonomy and phylogeny
Butterflies are divided into seven families that contain a total of about 20,000 species.
+ Butterfly families Family Common name Characteristics Image Hedylidae American moth-butterflies Small, brown, like geometrid moths; antennae not clubbed; long slim abdomen Hesperiidae Skippers Small, darting flight; clubs on antennae hooked backwards Lycaenidae Blues, coppers, hairstreaks Small, brightly coloured; often have false heads with eyespots and small tails resembling antennae Nymphalidae Brush-footed or four-footed butterflies Usually have reduced forelegs, so appear four-legged; often brightly coloured Papilionidae Swallowtails Often have 'tails' on wings; caterpillar generates foul taste with osmeterium organ; pupa supported by silk girdle Pieridae Whites and allies Mostly white, yellow or orange; some serious pests of Brassica; pupa supported by silk girdle Riodinidae Metalmarks Often have metallic spots on wings; often conspicuously coloured with black, orange and blue
Traditionally, butterflies were divided into the superfamilies Papilionoidea, Hesperioidea and the moth-like Hedyloidea. Recent work has discovered that Hesperiidae and Hedylidae, the only families within "Hesperioidea" and "Hedyloidea", are nested within the Papilionoidea, meaning that Papilionoidea would be synonymous with Rhopalocera. The relationships between the 7 families are extremely well resolved, which is summarised in the below cladogram.
Biology
General description
Butterfly adults are characterised by their four scale-covered wings, which give the Lepidoptera their name (Ancient Greek λεπίς lepís, scale + πτερόν pterón, wing). These scales give butterfly wings their colour; they are pigmented with melanins that give them blacks and browns, as well as uric acid derivatives and flavones that give them yellows, but many of the blues, greens, reds and iridescent colours are created by structural coloration produced by the micro-structures of the scales and hairs.
As in all insects, the body is divided into three sections: the head, thorax, and abdomen. The thorax is composed of three segments, each with a pair of legs. In most families of butterfly the antennae are clubbed (clavate), unlike those of moths which may be threadlike (filiform) or feathery (plumose). The long proboscis can be coiled when not in use for sipping nectar from flowers.
Nearly all butterflies are diurnal, have relatively bright colours, and hold their wings vertically above their bodies when at rest; these features differentiate butterflies from the majority of moths, which fly by night, are often cryptically coloured (well camouflaged), and either hold their wings flat (touching the surface on which the moth is standing) or fold them closely over their bodies. Some day-flying moths, such as the hummingbird hawk-moth, are exceptions to these rules.
Butterfly larvae, caterpillars, have a hard (sclerotised) head with strong mandibles used for cutting their food, most often leaves. They have cylindrical bodies, with ten segments to the abdomen, generally with short prolegs on segments 3–6 and 10; the three pairs of true legs on the thorax have five segments each. Many are well camouflaged; others are aposematic with bright colours and bristly projections containing toxic chemicals obtained from their food plants. The pupa or chrysalis, unlike that of moths, is not wrapped in a cocoon.
Many butterflies are sexually dimorphic. Most butterflies have the ZW sex-determination system, where females are the heterogametic sex (ZW) and males homogametic (ZZ).
Distribution and migration
Butterflies are distributed worldwide except Antarctica, totalling some 18,500 species. Of these, 775 are Nearctic; 7,700 Neotropical; 1,575 Palearctic; 3,650 Afrotropical; and 4,800 are distributed across the combined Oriental and Australian/Oceania regions. The monarch butterfly is native to the Americas, but in the nineteenth century or before, spread across the world, and is now found in Australia, New Zealand, other parts of Oceania, and the Iberian Peninsula. It is not clear how it dispersed; adults may have been blown by the wind or larvae or pupae may have been accidentally transported by humans, but the presence of suitable host plants in their new environment was a necessity for their successful establishment.
Many butterflies, such as the painted lady, monarch, and several danaine migrate for long distances. These migrations take place over a number of generations and no single individual completes the whole trip. The eastern North American population of monarchs can travel thousands of miles south-west to overwintering sites in Mexico. There is a reverse migration in the spring. It has recently been shown that the British painted lady undertakes a round trip in a series of steps by up to six successive generations, from tropical Africa to the Arctic Circle — almost double the length of the famous migrations undertaken by monarch. Spectacular large-scale migrations associated with the monsoon are seen in peninsular India. Migrations have been studied in more recent times using wing tags and also using stable hydrogen isotopes.
Butterflies navigate using a time-compensated sun compass. They can see polarised light and therefore orient even in cloudy conditions. The polarised light near the ultraviolet spectrum appears to be particularly important. Many migratory butterflies live in semi-arid areas where breeding seasons are short. The life histories of their host plants also influence butterfly behaviour.
Life cycle
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. The Melissa Arctic (Oeneis melissa) overwinters twice as a caterpillar. 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.
Courtship is often aerial and often involves pheromones. Butterflies then land on the ground or on a perch to mate. Copulation takes place tail-to-tail and may last from minutes to hours. Simple photoreceptor cells located at the genitals are important for this and other adult behaviours. The male passes a spermatophore to the female; to reduce sperm competition, he may cover her with his scent, or in some species such as the Apollos (Parnassius) plugs her genital opening to prevent her from mating again.
The vast majority of butterflies have a four-stage life cycle: egg, larva (caterpillar), pupa (chrysalis) and imago (adult). In the genera Colias, Erebia, Euchloe, and Parnassius, a small number of species are known that reproduce semi-parthenogenetically; when the female dies, a partially developed larva emerges from her abdomen.
Eggs
Butterfly eggs are protected by 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 fertilise the egg. Butterfly eggs vary greatly in size and shape between species, but are usually upright and finely sculptured. Some species lay eggs singly, others in batches. Many females produce between one hundred and two hundred eggs.
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 has been little researched but in the case of Pieris brassicae, it begins as a pale yellow granular secretion containing acidophilic proteins. This is viscous and darkens when exposed to air, becoming a water-insoluble, rubbery material which soon sets solid. Butterflies in the genus Agathymus do not fix their eggs to a leaf; instead, the newly laid eggs fall to the base of the plant.