Human Head Anatomy
The muscles of the head and neck perform many important tasks, including movement of the head and neck, chewing and swallowing, speech, facial expressions, and movement of the eyes. These diverse tasks require both strong, forceful movements and some of the fastest, finest, and most delicate adjustments in the entire human body.
The muscles of the face are unique among groups of muscles in the body. While most muscles connect to and move only bones, facial muscles mostly connect bones to skin. These muscles, including the zygomaticus major and orbicularis oris, pull on the skin to produce a seemingly infinite number of facial expressions and to move the lips and cheeks during speech and eating.
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| Vein structure of a transparent Satyrine butterfly |
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.
There a 3 basic types of scale - pigmentary scales, structural scales, and androconia.
1. 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.
2. 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 Morphobutterflies 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 causes 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.
3. Androconia scales:
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.
Web references:
http://www.innerbody.com/anatomy/muscular/head-neck
http://www.learnaboutbutterflies.com/Anatomy%203.htm
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