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Butterfly Anatomy
PAGE 3
PAGE 1 - HEAD
PAGE 2 -
THORAX / ABDOMEN
PAGE 3 -
WINGS
Wings
venation
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scales
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androconia
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hearing organs
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flight
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pattern
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thermo-regulation
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.
Venation
All butterflies
and moths ( except Plume moths ) have 2 pairs of
overlapping wings, each comprised of a very thin 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.
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| Vein structure
of a transparent Satyrine butterfly
Haetera piera (
photo courtesy Tony Hoare ) |
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 comprises of a flattened plate arising from a single cell
on the wing surface.
The scales vary
considerably in shape, some being rectangular, while others are
shaped like tear-drops or plumes. An individual scale might
typically measure about 50 microns across ( 1/20 of a millimetre )
and be 100 microns long, although many are hair-like, and are very
much longer.
There can be as many as 600 individual scales per sq millimetre of
wing surface, although in certain genera such as
Acraea, Aporia
and Parnassius the density is
considerably lower, giving the wings a translucent appearance. In
some tropical genera such as Ithomia,
Lamproptera and
Cithaerias the scales are absent from large areas of the
wings, resulting in almost complete transparency.
Catoblepia berecynthia
( Peru ), wing scales, magnification x10
There a 3 basic types of
scale - pigmentary scales, structural scales, and androconia.
Pigmentary scales
are mostly flat. Their colour is the result of the presence of
melanins, pterins and other chemical pigments, most of which are
sequestered from the larval foodplants and passed to the adult
butterflies. The pigments account for the basic colours found in
butterfly wings - black, red, yellow and white. The
juxtaposition of the various coloured scales, and the amount of
pigment they each contain, creates the illusion of
additional colours such as orange, cream and green.
In certain species such
as the Orange tip Anthocharis cardamines
the mottled green markings on the underside are an illusion caused
by having a finely balanced mixture of yellow and black scales.
Subtle variations in the
density and pigmentation of scales can create other illusions such
as texture or shading, which help to give the wings of some
butterflies a 3-dimensional appearance.
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.
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Structural scales, showing the fine ridges which diffract
light to create metallic colours
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Structural scales. Many of the most beautiful and striking
colours found on the wings of butterflies are created not by
pigments, but by
refraction,
or thin-film interference. This is comparable to
the way rainbow hues reflect from thin films of oil in puddles on a
rainy day. Examples of refractive colour include the purple sheen on
the wings of Apatura Purple Emperors,
and the greenish bloom seen on the wings of
Erebia Mountain Ringlets.
The really bright reflective colours however are produced
by a different means - diffraction. In this case light is broken up
into it's constituent colours after being diffracted by prismatic
ridges on the surface of the scales, or on a lattice of microscopic
bubbles within them.
Examples of
diffraction colouring include the fiery hues of
Lycaena Coppers, the golden-yellow of
Troides Birdwings, the glittering
metallic greens of Caria Metalmarks,
and
the almost blindingly reflective blues of the South American
Morpho butterflies.
Diffractive scales also usually exhibit a high degree of iridescence -
the
colours changing
in hue and intensity as light strikes the wings from different
angles. One
of the most extreme examples is the Sunset moth
Chrysiridia rhipheus, the slightest change of
angle causing metallic green bands on the fore-wings to change to
turquoise, while a contrasting patch on the hindwings undergoes an
even more dramatic change, cycling through every colour in the
rainbow as light hits it at different angles. Its extraordinary
brilliance and iridescence is due to its curved ribbon-like
scales, which cause light to bounce about between adjacent scales
rather than be reflected straight back to the observer.
Almost all
butterflies and moths have a mixture of pigmentary and structural
scales. In combination these can produce any colour ranging from
metallic gold to fluorescent orange,
iridescent green,
sapphire
blue,
or any other colour
seen on butterfly wings. They can even display colours beyond the
visible spectrum - most butterflies, in addition to the colours and
patterns visible to humans and birds, also have a "hidden"
ultra-violet pattern that can only be detected by other butterflies.
Androconia are
found mainly on male butterflies. They usually exist as slightly raised dark streaks or patches on the
forewings, and often have a mealy appearance. At the base of the
androconia are tiny sacs
containing scent ( pheromones ). The scent is disseminated via
tiny hairs or plumes on the edges of the scales, and used to
entice females to copulate.
Male androconia can also take
the form of tufts ( e.g. on the hindwings of Morpho
and
Charaxes, or can be found in androconial folds such as
found on the hindwings of Papilionidae or the costal fold of
Pyrginae. In the Danaini and Ithomiini they occur as "hair-pencils".
These can either take the form of extrusible organs at the tip of the abdomen, or
occur as long "hairs" on the hindwings. In some species
e.g. Lycorea the abdominal organ is
brushed against androconia on the hindwings to collect pheromones.
These are later disseminated by expanding the tufts in the
presence of females.
Androconia
can also occur as "stink-clubs" in the genital opening of female
Battus, Parides,
Troides, Ornithoptera and
Heliconius butterflies, and
in certain moth families e.g. Saturniidae, Lasiocampidae and
Lymantridae.
Gatekeeper
Pyronia tithonus, England. The
dark diagonal patch
on male's forewings 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. Males
habitually bask on tree trunks, where they wait to intercept passing
females. It has been speculated that the sound may deter
competing males from occupying the same territory, but I have
frequently found trees on which up to half a dozen males were
basking in close proximity. It seems more likely therefore that
the sounds act act
as a trigger to initiate responses from females during
courtship.
In
Morpho helenor the eardrum is located
at the base of the wing.
Photo
© Kathleen Lucas.
Kathleen Lucas of the University of
Bristol used a laser beam to scan the membrane of the eardrum of
Morpho peleides ( =
helenor ). She found that lower
frequencies between 1000 - 5000 Hz caused vibrations to focus on a
spot on the outer membrane, but that frequencies above 5000 Hz
caused the entire membrane to vibrate, including the "fried egg"
dome structure arrowed in the photo. Moth ears respond equally to
all frequencies, but Morpho
butterflies seem able to differentiate between low and high
pitched sounds. Lucas speculated that this could help the
butterflies figure out if birds are about to attack. If e.g. they
could tell apart the sounds of flapping bird wings and
those of bird song, it might trigger different escape responses by
the butterfly.
Some scientists
believe that when butterflies first evolved they were nocturnal,
and that their 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 avoid hitting unseen obstacles.
Certain moths, particularly among the Noctuidae, are able to hear the bat's acoustic pulses, and react extremely
swiftly, swerving or dropping to avoid the
unseen approaching predator.
Nerve
cells similar to those in the "ears" are also found in enlarged veins at the base of the fore-wings of many butterflies.
These are particularly well developed in Satyrines such
as
Oressinoma, 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 such as
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 species adopt a "perch and wait" mate locating
strategy, and need to be able to take flight rapidly to intercept
potential mates. Examples include
Skippers ( Hesperiinae ),
Metalmarks
( Riodinidae ), and
Graylings ( Satyrinae ).
These species often tend to have triangular 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 ), Blues (
Lycaenidae ) and Morphos ( Morphini ) adopt a
"patrolling" mate location strategy. Thus they have no need for such rapid
acceleration. They tend therefore to have rounder and less robust wings,
which are larger in relation to their thinner and less
muscular bodies.
Consequently their flight is much lazier.
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Eurybia species, probably
molochina, Madre de Dios, Peru
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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 dash 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
wing patterns of butterflies, even those which appear to be
radically different, have been analysed and demonstrated by
Schwanwitsch to follow a common ground plan. This involves various
concentric and radiating bands, the juxtaposition of which can
generate anything from the broad swathe of blue on a
Doxocopa to the series of ocelli around
the margins of a Euptychia or the
radiating red streaks on the hindwings of Heliconius erato. There are almost certainly at least 5 such
ground plans - one each for the Papilionidae, Pieridae, Hesperiidae,
Nymphalidae and Lycaenidae.
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 to use behavioural strategies
instead.
In
cool conditions butterflies need to raise their body temperatures
before they are able to 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 & 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.
Small Heath Coenonympha pamphilus "lateral basking" on a stone
The
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 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,
including Peacocks, Small Tortoiseshells & Red Admirals prepare 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 this activity 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 often fly into bushes to seek shade, or enter rabbit burrows.
Plume moths
Plume moths ( Pterophorinae ) and Many-plumed moths (
Alucitidae ) have no wing membranes.
Instead their fore and hind wings each consist of plumes - rigid spines from which branch dozens of long thin
feathery scales.
There are 186 known species of Alucitidae worldwide, many of
which have only been discovered in the last 20 years. The name
of the moth illustrated below, Alucita hexadactyla
translates as "20 fingers" and is a misnomer - Alucitidae
actually have 24 of these feathery fingers, although some are hidden from view
in the photo.
Many-plumed moth
Alucita hexadactyla,
Hampshire, England
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