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Butterfly Anatomy PAGE 1

PAGE 1 - HEAD
PAGE 2 - THORAX / ABDOMEN
PAGE 3 - WINGS
 
Head
 
eyes | antennae | Johnston's organ | labial palpi | proboscis | feeding
 
 
 
 
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 small 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 narrow angle seen by each ommatidium means that a detailed image of their surroundings is probably only possible at very close distances when the individual elements merge together.
 
The lenses cannot focus, but due to their extremely short focal length they don't need to - the laws of optics mean that everything between about 5mm to 100 metres is probably rendered sharply.
 
Unlike vertebrates, which need to move their eyes / heads to scan their surroundings, butterflies have almost 360 degree vision. They can see everything at the same time, e.g. they can accurately probe into flowers in front of them, and at the same time devote equal concentration to detecting threats from behind.
 
The butterfly's brain can instantly detect whether the image formed by each ommatidium is dark or light. If a predator approaches, the amount of light hitting each receptor changes instantly because of it's narrow angle of view. This, in combination with binocular vision, means that the compound eyes are extremely efficient at detecting the movement and distance of approaching predators, enabling them to take immediate evasive action.
 
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.
 

Humans and birds perceive colours and patterns in a different way to butterflies, as the latter have the ability to detect ultra-violet as well as visible radiation. Flowers have ultra-violet patterns that are invisible to humans but can be recognised by butterflies. The patterns guide the butterflies to the source of nectar in much the same way that runway lights guide an aircraft in to land.

 

The wings of butterflies have visible patterns that can be seen by mammals and birds, but they also have separate ultra-violet patterns which can only be detected by other butterflies. These allow them to quickly distinguish their own species from others with similar visible patterns.

Visual selection of plants for egg-laying

At Stansted Forest in May 2008, I watched a female Green-veined White as she spent several minutes searching for places to lay her eggs. Every 4 - 5 seconds she would alight momentarily on a leaf, "tasting" it using olfactory sensors on her feet to check whether or not it was the correct foodplant for her offspring.

Surprisingly she tested bracken, ivy and oak leaves ( all visually very different from the crucifers she needed to locate for oviposition ), a fact which appears to indicate that sight plays little or no role in selecting plants for egg-laying.

 
How well can moths see at night ?
 
Elephant Hawkmoths have been studied to determine whether or not nocturnal moths can perceive colour. Kelber et al found that this species has 9 light sensors in each ommatidium ( compared to between 2 - 6 in butterflies ); and used behavioural experiments to prove that the moths are able to discriminate coloured stimuli at intensities corresponding to dim starlight.
Why do some butterfly species have hairy eyes ?
All butterflies in the genus Lethe ( Satyrinae ) have a dense layer of fine bristles or "hairs" on their compound eyes. My observations of various Lethe species in Sri Lanka, Borneo and West Malaysia indicate that the adults are strongly attracted to wet dung, and spend long periods probing into it, at which times their heads push right into the substance. It seems possible therefore that the "hairs" may function in the same way as a cat's whiskers, acting as tactile sensors which warn the butterfly if their eyes get too close to the dung, which would almost certainly ( temporarily or permanently ) blind them if it stuck to the surface of the eyes.
 
Bear in mind however that not everything in nature has a "purpose" or "reason". It could simply be the case that the hairs first appeared as a random mutation that was neither beneficial or harmful, and consequently there would be no natural selection pressure for it to "breed out" and revert to a non-hairy eye.
Antennae
 
From between the eyes emerge a pair of segmented antennae. These can be angled at various positions, and are best thought of as a form of radar - they are used to detect pheromones ( scents used for mate location and selection ), and guide the butterfly towards potential mates.
 
Essex Skipper Thymelicus lineola ( England ) frontal view of antennae.
 
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 - I have often watched male Small Tortoiseshells Aglais urticae drumming their antennae on the hindwings of females during courtship, possibly to "taste" pheromones on the female's wings.
 
It is also common to see butterflies "antenna dipping" - dabbing the tips of their antennae onto soil or leaves. In this instance they are sampling the substrate to detect it's chemical qualities. Males use this method to establish whether soil contains essential nutrients ( males of many species drink mineralised moisture to obtain sodium ). Female butterflies use the same technique to assess the chemical qualities of foliage, which helps them determine whether the plant is of the correct species for egg-laying.
 
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 in many species are hooked at the tip, but most other butterflies have rounded ends to the clubs.
 

Some moths, such as the Burnets ( Zygaenidae ) and Cane Borers ( Castniidae ) have antennae that are clubbed, much like those of butterflies. This is one of several reasons why any distinction between butterflies and moths is difficult to define scientifically.

6-spot Burnet Zygaena filipendulae ( Zygaenidae ), England. Burnet moths have antennae that are clubbed even more than those of true butterflies.

Male moths from the Saturniidae, Lasiocampidae and many other families have strongly feathered 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.

The antennae of the male Fox moth Macrothylacia rubi are strongly feathered

Most other moths have very thin tapering antennae. In some such as the tiny day-flying Nemophora degeerella from England, these are incredibly long, but their function appears to be unknown.

Nemophora degeerella has antennae 5 times the length of it's wings

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 likely that they are able to detect magnetic fields when migrating.

Labial 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 attuned to the detection of 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 wipers to clean 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, but this theory certainly does not hold true for the Beak butterflies ( Libytheinae ) which have extremely long palpi but from my observations feed mainly on mineralised moisture at the edge of puddles.
 

Beak butterfly Libythea myrrha ( Malaysia ), showing labial palpi projecting from head.

Proboscis

The proboscis consists of a pair of interlocking 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 into flowers to suck up the liquids on which they feed. 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.
 

The "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.
 

Feeding behaviour

In temperate zones most butterflies obtain their sustenance from flowers, but there are exceptions - male Purple Emperors for example never visits flowers; they live entirely on fluids which they obtain from dung, carrion, urine-soaked ground, tree sap, and on "honey dew" - sugary aphid secretions which coat the upper surface of tree foliage in mid-summer.
 
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 many 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 females of Heliconius Longwings visit Lantana and various other flowers for nectar, and sequester pollen from Psiguria flowers in the rainforest. Individual females have the ability to learn and remember the location of particular Psiguria plants. They visit these every day, following a predefined circuit through the forest. The pollen collected from the flowers is processed by the females to extract proteins which enable them to produce eggs for up to 9 months.
 
Often dozens or even hundreds of butterflies ( e.g. males of Eurema, Phoebis, Pyrgus, Trogonoptera Marpesia, Adelpha, Callicore, Graphium & Eunica ) gather on river sandbanks to filter-feed, drinking mineralised water from puddles or damp sand. Many other species such as Doxocopa, Perisama and Lymanopoda also gather in lesser numbers to imbibe moisture in the same way.
 
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 once saw 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 metalmark feeding on the corpse of a bullfrog.
 
In temperate regions carrion-feeding is less common than in the tropics, but I once found 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 Ithomiines ( Tiger-mimics, Glasswings ) and Skippers form associations with ant-bird colonies. The birds follow marauding soldier ant armies, feeding on insects which scatter as the ants approach. In turn the Skippers and Ithomiines follow the ant-birds, feeding on their liquefied droppings.
 
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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