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Text and photographs protected by Copyright © Adrian Hoskins 2007, and must not be published in part or in whole elsewhere without prior written permission from the author.
Butterfly Anatomy PAGE 3
 
PAGE 1 - HEAD
PAGE 2 - THORAX
PAGE 3 - WINGS
 
 
 
Wings
 
venation | scales | androconia | hearing organs | flight | pattern | thermo-regulation
 
Venation
 
All butterflies, and most* moths, have 2 pairs of overlapping wings, each comprised of a double membrane, with rigidity supplied by a network of tubular veins which radiate from the base of the wings. The pattern of veins is different for every genus of butterfly, and is one of the main criteria used by taxonomists when classifying butterflies.
 
 
A female Brimstone Gonepteryx rhamni, seen here extending it's proboscis to suck up nectar from a thistle flower.  The pattern of raised wing veins ( venation ) can be seen clearly.
 

 *
The exceptions to this rule are the Plume moths ( Pterophorinae ) and Many-plumed moths
( Alucitidae ). In these, the wings are split into groups of between 3 and 7 "feathers", each formed from a single vein which is adorned with a fringe of long hair-like scales.
Also, the females of some moths in the families Geometridae and Lymantridae are wingless. Thus dispersal and expansion of range is dependent on the mobility of the caterpillars.

Scales

The wing membranes are transparent, but are partially or fully covered in a dust-like layer of tiny coloured scales, each overlapping it's neighbour like the tiles on a roof. Each scale consists of a flattened plate extending from a single cell on the wing surface, and contains pigments which are derived from chemicals in the caterpillar's foodplants.
Catoblepia berecynthia ( Peru ), wing scales, highly magnified

Each species has only 4 or 5 different coloured scales. The varying proportions of these basic colours can create the illusion of further colours, and their arrangement creates the extraordinary variety of patterns found on butterfly's wings.
The beautiful patterns on a butterflies wings are made up from only 4 or 5 basic colours, but the proportions and arrangements of these hues creates the illusion of many more colours.
 
 
Many butterflies exhibit brilliant iridescent colours - blue, purple, copper, green, silver and gold, which change in hue and intensity as light strikes the wings from different angles. These colours are not produced by pigments, but by the reflection or refraction of light, acting either on prismatic ridges on the surface of the scales, or on a lattice of microscopic bubbles within them.
 
 
The refracting wing scales of the neotropical ringlet Caeruleuptychia lobelia are arranged in a concentric pattern on the wing surface.
 
Androconia
 
Males of many species have specialised scales on their forewings called "androconia", which have sacs at their bases containing scent ( pheromones ). The pheromones are disseminated into the atmosphere via tiny hairs or plumes on the edges of the scales, and are used to entice females to copulate.
 
In the Danainae the androconia are on the hindwings. The males are equipped with tufts of  hair-pencils at the tips of their abdomens, which they brush against the scales to collect the pheromones, and these are later disseminated by expanding the tufts when in the presence of females.
 
Gatekeeper Pyronia tithonus, England. The dark diagonal patch on the forewings of the male are composed of hundreds of androconial scales. These disseminate pheromones that can be detected by females during courtship. As the male ages the strength of his pheromones diminishes, thus by analysing the strength of the pheromones a female can assess the age and virility of a potential mate.

Hearing organs
Some butterflies, including the Hamadryas Crackers and Heliconius Longwings can detect sound, using an "ear" near the base of the underside of their wings. The ear can only be seen with the aid of a powerful microscope. It takes the form of a funnel shaped sac, covered with a very thin membrane. This vibrates in response to high frequency sound, and stimulates nerve cells called scolopidia, which send a message to the butterfly's brain.
 
Hamadryas butterflies use their ears to detect crackling noises made by territorial males. The sound is made by twanging 2 tiny prongs on the tip of the abdomen against bristles on the valvae. The males habitually bask on tree trunks, where they wait to intercept passing females. The sound produced by the males probably serves to deter competing males from occupying the same territory, and may also act as a trigger to initiate the first response from a female during courtship.
 

 

 

The "ear" on a butterfly's wing, photographed by an electron scanning microscope.

 

 

 
 
 
 
 

Some scientists believe that when butterflies first evolved they were nocturnal, and that the ears originally served to detect and avoid predatory bats. Bats emit acoustic pulses when flying at night, and use their highly sensitive ears to detect the echo reflected back by solid objects. This way they can avoid hitting unseen obstacles. Certain moths, particularly the Owlets and Underwings ( Noctuidae ) are able to hear the bat's acoustic pulses, and react extremely swiftly, using tactics such as swerving or dropping to avoid the unseen approaching predator.

Nerve cells similar to those in the "ears" are also found in the enlarged veins at the base of the forewings of many butterflies. This appears to be particularly well developed in Satyrines such as Maniola, Pararge and Hipparchia, all of which react instantly to the sound made as dry leaves are crunched underfoot, or to the noise made by the shutter of a camera.

Flight

Skippers tend to have a buzzing moth-like flight, and other small butterflies such as Lycaenids and Riodinids need to beat their wings rapidly to propel themselves through the air. Larger species - Nymphalids, Pierids and Papilionids - fly by a combination of flapping and gliding. When gliding, the wings are held so as to create a concave under-surface, producing a parachute effect which slows the rate of descent. These larger species also make use of thermals to gain or maintain height when gliding above the forest canopy, or when migrating.

Males of many butterflies adopt a "perch and wait" mate locating strategy, and need to be able to take flight rapidly to intercept potential mates. Examples include Hairstreaks

( Theclinae ), Pashas ( Charaxinae ) and Skippers ( Hesperiidae ). These species often tend to have pointed forewings with a particularly thick and straight costa. The springy qualities of the costa, in combination with their powerful flight muscles, enables them to accelerate rapidly at take off.

Other species, such as Whites ( Pierinae ), Swallowtails ( Papilionidae ) and Browns
( Satyrinae ) adopt a "patrolling" mate location strategy. Thus they have no need for such rapid acceleration. They generally have rounder and less robust wings, which tend to be large in relation to their thinner and less muscular bodies. Consequently their flight is much lazier.
 
Eurybia species, probably molochina, Madre de Dios, Peru
 

In the neotropics, Eurybia butterflies ( Riodinidae ) habitually spend long periods resting upside down and with wings spread open, beneath the leaves of low growing vegetation. Flight analysis has shown that by doing so they are able to take off much more rapidly than they could if they rested the "right" way up. From their hiding place they keep a watchful eye on passing insects. Periodically they fly out to intercept and investigate other butterflies, but instantly return to settle under a nearby leaf. The speed of flight is remarkable, and the degree of agility apparent when they fly into the vegetation, flip upside-down and settle under another leaf is quite amazing to behold.

  
 
Colour and pattern
 
The wings of a butterfly are not simply used for flight. Their colours and patterns serve to identify them to potential mates, to startle or warn predators, and to provide them with camouflage or disguise when at rest.
 
The use of colour and pattern is discussed in detail in the Survival Strategies section.
 
Thermo-regulation
 
Butterflies are cold-blooded. If they are too cold they cannot fly. If they get too hot they become dehydrated and die. They have no internal means of regulating their body temperature, so they need use behavioural strategies instead.
 
In cool conditions butterflies need to raise their body temperatures before they are able fly. To do so they use a technique known as dorsal basking, whereby they use the upper surface of their wings as solar panels to absorb heat and give them energy. Often they settle to bask on pale, heat-reflecting substrates such as stones, tree-trunks or patches of bare ground. Heat is reflected back from the substrate and absorbed by the dark undersides of the wings, speeding up the warm-up process. Males in particular use this method, to ensure that they always have sufficient energy available to enable them to instantly fly up to intercept passing females.
 
Red Admiral Vanessa atalanta basking on a tree trunk on a cold but sunny winter day
 
Some butterflies, such as Clouded Yellows, Graylings and Green Hairstreaks, always keep their wings closed when at rest, and adopt another technique known as lateral basking. In cool conditions they bask by tilting their wings over to one side, so as to present the maximum area of wing surface to the sun. Conversely, when they get too hot, they tilt in the opposite direction so that their wing surfaces are parallel to the sun's rays, and present the minimum surface area to the sun.
 
Grayling Hipparchia semele "lateral basking" on a stone
 
Whites, Blues and Coppers have wing surfaces which reflect, rather than absorb solar energy. Consequently they bask with their wings half open, so that the heat produced by sunlight falling on the dark thorax is contained within the "cage" of the half-open wings, rather than being dispersed on the breeze. This behaviour is called reflectance basking.
 
Hesperiine skippers such as Ochlodes venata also adopt a similar strategy, basking with their hindwings outspread, but their forewings raised at 45 degrees. Skippers have relatively small wings and thick heavy bodies, so they need to beat their wings more rapidly than other butterflies. Using reflectance basking enables them to raise their body temperatures quickly to a level that allows them to fly.
 
Large Skipper Ochlodes venata "reflectance basking"
 
Another method used to raise body temperatures is "shivering". Many Nymphalid species, such as Peacocks, Large Tortoiseshells and Camberwell Beauties ready themselves for flight by rapidly shivering the wings ( which are held closed during this process ). Even on the coolest day, a minute or two of wing-shivering generates enough friction to warm the body enough to enable them to fly short distances.
 
Butterflies can only operate within a limited temperature range, so on hot days they need to find ways of keeping cool. Forest-dwelling species simply hide beneath leaves, while species that inhabit open areas such as grassland and heaths often fly into bushes to seek shade, or enter rabbit burrows.

 

 
Related subjects :
 
Taxonomy  - the classification and characteristics of butterfly families and sub-families.
Lifecycle - egg, caterpillar, chrysalis, adult butterfly, courtship, reproduction.
 
Text and photographs protected by Copyright © Adrian Hoskins 2007-2008, and must not be reproduced or published in part or in whole elsewhere in any form without written permission from Adrian Hoskins. Breach of copyright will be pursued by litigation.
 
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