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Butterfly Anatomy
Page 3
1 - Head
2 -
Thorax, legs & abdomen
3 -
Wings -
venation & scales
4
- Wing scales - scanning electron microscope images
5
- Hearing organs, flight,
thermoregulation
Wings
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.
Vein structure
of a transparent Satyrine butterfly
Haetera piera (
image © Tony Hoare )
Scales
The wing
membranes are transparent, but are partially or fully covered in a
dust-like layer
of tiny coloured scales. Each scale comprises of a flat 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 and yellow. The
juxtaposition of the various coloured scales, and the amount of
pigment they each contain, can create the illusion of
additional colours such as orange, cream and green.
In some 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 scale pigmentation and density can create
illusions such as texture or shading, which help to give the wings
of some butterflies a 3-dimensional appearance.
Pigmentary
scales are also known as "ground scales" as they effectively form a
lower ground layer of colour and pattern on a butterfly's wings. The
scales are laid out in neat rows like the tiles on a roof. Each row
comprises of alternating pigmentary and structural "cover scales".
The latter are larger than the pigmentary scales. They overlap them,
and are semi-transparent so the colours of the pigmentary scales can
be seen through them.
Structural scales. The
fiery hues of
Lycaena Coppers, the golden-yellow of
Troides Birdwings, the glittering
metallic greens of Caria Metalmarks
and
the dazzling blues of the South American
Morpho butterflies are produced by the
refraction, diffraction and interference patterns of light as it
strikes or passes through the semi-transparent structural scales.
In the
case of diffraction
light is
broken up into lighter or darker bands after passing through a
lattice of microscopic bubbles or slits within the scales.
Refraction on the other hand is where light is broken up into
its constituent rainbow colours as a result of passing through
prismatic ridges on the surface of the scale. Interference
patterns are the result of light passing through clear layers of
varying density, and being reflected back in such a way that the
colours change according to the angle of view. Examples of such
iridescent colour are found in many butterflies, but are
particularly striking in neotropical Doxocopa
butterflies, where a band of colour can change from electric blue to
vivid turquoise or dazzling silver as sunlight strikes the wings at
different angles.
Another
example is the Sunset moth
Chrysiridia rhipheus, in which 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.
Large Skipper
Ochlodes venata, England.
The
dark diagonal patch
on forewings of the
male Large Skipper illustrated above 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.
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 species of Alucitidae worldwide, many of which
have only been discovered in the last 20 years. The name of the
moth shown below, Alucita hexadactyla
translates as "20 fingers" but is a misnomer - Alucitidae
actually have 24 feathery plumes ( some are hidden in the image
below ).
Many-plumed moth
Alucita hexadactyla,
Hampshire, England
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