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Butterfly Anatomy

1 - Head
2 - Thorax - legs & abdomen
3 - Wings - venation & scales
4 - Wing scales - scanning electron microscope images
5 - Hearing organs, flight, thermoregulation
Head
Antennae, eyes, palpi, proboscis

Eyes

Butterflies and most other adult insects have a pair of spherical compound eyes, each comprising of up to 17000 "ommatidia" - individual light receptors with their own microscopic lenses. These work in unison to produce a mosaic view of the scene around them.

Structure

Each ommatidium consists of a cornea and cone, which together function as a lens. Emerging from the back of each cone is a rod down which light travels to reach a cluster of 2-6 sensory cells, each of which is sensitive to a particular part of the visual spectrum.
The eyes of Skippers are different from those of other butterflies. They have a space between the cones and rods which allows light from each ommatidium to spill into neighbouring rods, effectively increasing their resolution and sensitivity. As a result Skippers can fly very accurately from one spot to another. This different type of eye structure is one of the reasons why taxonomists place them in a different super-family to all other butterflies - the Hesperioidea.
Capabilities
The laws of optics show that it's likely that everything from about one centimetre to 200 metres will be rendered in sharp focus by butterflies, as their ommatidia are of very short focal length.
The butterfly's brain can instantly detect whether the image formed by each ommatidium is dark or light. If a predator approaches or if the butterfly moves its head a tiny fraction, the amount of light hitting each receptor changes instantly because of it's very narrow angle of view. This sensitivity to changes in its surroundings means that a butterfly is extremely efficient at detecting movement and at gauging the distance of an approaching predator, enabling it to take immediate evasive action.
The sensitivity to changes in their visual field, combined with a high flicker-vision frequency of about 150 images per second, may also help butterflies to piece together the thousands of elements of the mosaic image produced by the compound eye. It is not known whether butterflies and other insects are able to merge these mosaic elements into a single image. If are able to do so, it would render them capable of distinguishing patterns at close distances.
Vertebrates need to move their eyes and heads to scan their surroundings, but the compound eyes of butterflies provide them with almost 360 degree vision. They can see everything at the same time, so they can accurately probe into flowers in front of them, and at the same time devote equal concentration to detecting threats from behind.
Butterflies can see polarized light, enabling them to determine the position of the sun, even when it is partly hidden by cloud. This lets them relate their position to the sun and use it as a compass when moving around their habitats.
Colour perception
Humans and birds perceive colours in a different way to butterflies, as the latter are ultra-sensitive to UV as well as visible radiation. Flowers have ultra-violet patterns that are invisible to humans but which can be recognised by butterflies. These UV patterns guide butterflies to the source of nectar in much the same way that runway lights guide an aircraft in to land.
Experiments on Colias butterflies dyed orange, red, green, blue and black have shown that females don't discriminate between males of different colours. Most biologists agree that visible colours and patterns are NOT used for butterfly-to-butterfly communication. Their primary function is to convey survival-related signals to birds ( i.e. camouflage, aposematic colour, mimetic patterns etc ).
Butterflies can communicate with each-other visually, but they use a "private channel" of ultraviolet patterns which are overlaid on the visible patterns, and cannot be seen by vertebrates. They enable butterflies to recognise conspecifics during the initial "approach" phase of mate location. It has been proven by experimentation that males which have had their UV-reflecting patterns obliterated suffer a significant drop in mate-location success.
As well as being sensitive to UV patterns, butterflies are also alert to the iridescent colours produced when sunlight refracts from the wings of other butterflies. Many species have also evolved selective colour response, i.e. they are "tuned" to react to colours that are dominant in the wing patterns of their own species. Examples include Heliconius erato which is sensitive to red, Morpho helenor which reacts very strongly to blue, and Philaethria dido which is responsive to green.
Shape perception
Male butterflies will intercept and chase any insect of approximately the same size and colour as the female of their own species during the approach phase of mate-location. Experiments using dummy cardboard females have however shown that males respond equally to square, circular, triangular, or butterfly-shaped dummies.
Females of some species however seem capable of recognising plants purely on the basis of leaf-shape and colour. This ability varies from one species to another, and is most highly developed in monophagous butterflies - those whose larvae will only eat one type of plant.
Polyphagous butterflies ( those which utilise several families or genera of larval foodplant ) tend to rely almost exclusively on chemical cues. I have e.g. often observed Pieris napi females searching for oviposition sites. They alight momentarily on various plants, sampling each by puncturing the leaf cuticle with spurs on the legs, to release chemicals in the leaf which are then tasted using the olfactory receptors in the feet. Leaves which were tested included bracken, ivy and oak leaves, all of which are very different in shape from the crucifers needed for oviposition. This appears to indicate that in this species sight plays little or no role in selecting plants for egg-laying.
Vision in nocturnal moths
Elephant Hawkmoths Deilephila elpenor have been studied to determine whether or not nocturnal moths can perceive colour. It seemed unlikely, but Kelber et al found that this species has 9 light receptors in each ommatidium ( compared to between 2-6 in butterflies ); and used behavioural experiments to prove that the moths can discriminate coloured stimuli at intensities corresponding to dim starlight.
Optical maintenance
Insects are unable to blink, so need other ways to protect their eyes. In many butterflies and moths the eyes are shielded by the labial palpi, which act as dust filters. Butterflies in the Satyrine genus Lethe have a dense layer of fine setae or "hairs" on their compound eyes. Studies by the author of these butterflies in Sri Lanka and Borneo indicate that they are strongly attracted to wet dung, and spend long periods probing into it. It seems plausible therefore that the setae could function in the same way as a cat's whiskers, acting as tactile sensors that warn them when their eyes get too close to the dung, which would blind them if it stuck to the eye surface.
Antennae
From between the eyes emerge a pair of segmented antennae. These can be voluntarily angled at various positions, and are best thought of as a form of radar. They have many functions including  pheromone detection, which is used for mate location and recognition.
Essex Skipper Thymelicus lineola ( England ) frontal view of antennae  Adrian Hoskins
The antennae of Monarchs Danaus plexippus are covered in over 16000 olfactory ( scent detecting ) sensors - some scale-like, others in the form of hairs or olfactory pits. The scale-like sensors, which number about 13700 in total, are sensitive to sexual pheromones, and to the honey odour which enables them to locate sources of nectar.
Butterfly antennae, like those of ants and bees may also used to communicate physically - e.g. it is common to see male Small Tortoiseshells Aglais urticae drumming their antennae on the hindwings of females during courtship, possibly to "taste" pheromones on the female's wings. Similar activity can be found in Wood Whites Leptidea sinapis and many other species.
Butterflies are often observed "antenna dipping" - dabbing the antennal tips onto soil or leaves. In this case they are sampling the substrate to detect it's chemical qualities. They do this to establish whether soil contains essential nutrients. Male butterflies often drink mineralised moisture to obtain sodium, which they pass to the females during copulation.
Differences between butterfly and moth antennae
Butterfly antennae are always clubbed at the tips. In most butterfly subfamilies e.g. Nymphalinae, Heliconiinae and Pierinae the shaft is straight and the club very pronounced, but in the Ithomiinae the antennae thicken progressively towards the tip. The clubs of Skippers ( Hesperiidae ) taper to a fine point and are hooked at the tip, but most other butterflies have rounded ends to the clubs.
Some moths including Burnets ( Zygaenidae ) and Cane Borers ( Castniidae ) also have antennae that are clubbed just like those of butterflies. This is one of many reasons why the "convenience" division of Lepidoptera into butterflies and moths is difficult to justify scientifically.
6-spot Burnet Zygaena filipendulae ( Zygaenidae ), England. Burnet moths have antennae that are clubbed even more than those of true butterflies  Adrian Hoskins
Male moths from the Saturniidae, Lasiocampidae and a few other families have plumed "pectinate" antennae which are covered in tens of thousands of olfactory sensors, and can detect the scent of females from distances of up to 2km away. The females have no need to detect pheromones, so their antennae, although similar in structure, have very much shorter plumes.

Antenna of male American Oak Silkmoth Antheraea polyphemus Emily Halsey

Johnston's organ

At the base of the antennae is a "Johnston's organ". This is covered in nerve cells called scolopidia, which are sensitive to stretch, and are used to detect the position of the antennae, as affected by gravity and wind. Thus they are used to sense orientation and balance during flight, and enable the butterflies to finely adjust their direction or rate of ascent / descent. It is also thought possible that they are able to detect magnetic fields when migrating.

Palpi

Protruding from the front of the head are a pair of small projections called labial palpi, which are covered in olfactory ( scent detecting ) sensors. Similar sensors are also located on the antennae, thorax, abdomen and legs.

These sensors are present in a variety of forms, and it is likely that each type fulfils a different role. Sensors on the antennae for example might be "tuned" to locate sexual pheromones, while those on the legs may be sensitive to chemicals exuded by larval foodplants. Logic would indicate that those on the labial palpi and proboscis, due to their position, might be tuned to detect adult food sources such as nectar, urine, carrion or tree sap.

Alternatively it is possible that they might function to detect the "smell" of air which emanates from particular locations - incoming dry desert air for example might be detected and act as a trigger to stimulate migration.

Some biologists argue that in addition to their olfactory functions, palpi have other functions such as shielding the proboscis. Logically this would mean a short proboscis would be associated with small palpi, and a long proboscis associated with larger palpi. In fact this is not the case - species with very long proboscises, such as Saliana skippers and Eurybia Underleafs have average sized palpi, while Libythea Beaks and other species with prominent palpi have unremarkable proboscises.

Another theory is that the palpi may serve as dust filters to protect the surface of the eyes. DeVries states that the most well developed palpi are found in butterflies which feed as adults on rotting fruit or dung where there is a greater probability of soiling the eyes or becoming infested with mites. This theory however doesn't hold true for Libythea Beak butterflies which have extremely long palpi but which feed at flowers, or in the case of males at mineralised moisture at the edge of puddles.

Beak butterfly Libythea myrrha, showing labial palpi projecting from head  Adrian Hoskins

Proboscis
The proboscis consists of a pair of interlocking c-section channels that when linked together form a tube, much like a drinking straw. This tube can be coiled up like a spring for storage, or extended to enable the butterfly to reach deep into flowers to suck up nectar. If the proboscis gets clogged with sticky fluids the 2 sections can be uncoupled and cleaned.
Olfactory sensors near the tip of the proboscis and in the food canal, together with similar sensors on the tarsus and tibia of the legs, enable butterflies to "taste" nectar, pollen, dung and minerals.
"BD" butterfly Callicore cynosura, using its proboscis as a drinking straw to imbibe dissolved minerals from the surface of a damp rock on the shore of an Amazonian tributary  Adrian Hoskins
Feeding behaviour
In temperate zones most butterflies obtain their sustenance by sucking nectar from flowers. There are exceptions however - male Purple Emperors Apatura iris for example never visits flowers; they feed entirely on fluids which they obtain from sources including dung, carrion, urine-soaked ground, tree sap and honeydew ( aphid secretions ).
In the Alps and Pyrenees mountain ranges of Europe males of many species, particularly Lysandra, Pyrgus, Thymelicus, Cupido & Mellicta often aggregate in groups of several dozen ( and sometimes several hundred ) to imbibe mineralised moisture from the edges of puddles, urine-soaked ground or cattle dung. This phenomenon is common in alpine regions throughout the northern hemisphere. 
In the tropics the majority of males from all families follow the behaviour described above for the Purple Emperor. Females of some species appear not to feed at all, and rely on proteins and amino acids transferred via the sperm of males during copulation. In the case of Papilionidae, Pieridae and Lycaenidae however females commonly obtain sustenance from flower nectar. 
In Central & South America female Heliconius butterflies visit Lantana and various other flowers for nectar. They also sequester pollen from Psiguria, Anguria and Gurania flowers in the rainforest. The pollen collected from the flowers is processed by the females to extract amino acids which increase longevity and enable them to produce eggs for up to 9 months. The butterflies have acquired the ability to learn and remember the locations of individual pollen plants. They visit these every day, following a predefined circuit through the forest.
Phoebis argante and Rhabdodryas trite aggregating to imbibe moisture, Peru  Adrian Hoskins
Swarms of butterflies, e.g. males of Eurema, Phoebis, Marpesia, Adelpha and Callicore habitually aggregate on river beaches to filter-feed, drinking mineralised water from damp sand. Numerous other species such as Doxocopa, Rhetus & Caria also gather in lesser numbers in similar situations.
Males from subfamilies such as Charaxinae and Apaturinae are commonly attracted to dung, rotting fruit or carrion. DeVries has estimated that at least 40 percent of all Nymphalidae in Costa Rica feed exclusively on rotting fruit.
The carrion feeders vary enormously in their choice of foodstuff. In Ecuador I have commonly seen Glasswings feeding on the decomposing corpses of robber flies, and in Venezuela I watched a male Rhetus periander sucking fluids from the corpse of a giant tarantula. At Pululuhua Crater in Ecuador I once found scores of high-altitude Satyrines including Lymanopoda, Lasiophila & Junea feeding on a snake corpse; and at Maquipucuna Cloudforest I stumbled upon a stunning Necyria avidly feeding on the corpse of a bullfrog.
In temperate regions carrion-feeding is far less common than in the tropics, but I fondly remember finding 6 male Purple Emperors Apatura iris feeding at the carcass of a deer that was floating in an open cesspit in a thicket in southern England. The butterflies were so stupefied by their unsavoury meal that 2 of them remained on the carcass as I lassoed a rope around the antlers and hauled it to the edge of the cesspit to take photographs !
In the rainforests of South America many butterflies form associations with ant-bird colonies. The birds follow armies of marauding soldier ants, feeding on insects that scatter as the ants approach. In turn the butterflies follow the ant-birds, feeding on their liquefied droppings. Biologists studying butterflies in rainforests commonly place tiny wads of dampened white tissue, designed to simulate bird-droppings, on leaves to attract butterflies from the families Hesperiinae and Ithomiinae.
The feeding behaviour of butterflies is discussed in greater detail in the individual species accounts, which can be accessed from the galleries or the Species Index.
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