Scale (Lepidopteran anatomy)

The presence of scales on the wings of Lepidoptera, comprising moths and butterflies, characterises this order of insects. The name is derived from Ancient Greek (scale) and (wing). The wings of Lepidoptera are minutely scaled, which feature gives the name to this order. Scales also cover the head, parts of the thorax and abdomen as well as parts of the genitalia.

The morphology of scales has been studied by Downey & Allyn (1975) and scales have been classified into three groups, namely:

Primitive moths (non-Glossata and Eriocranidae) have 'solid' scales which are imperforate, i.e., they lack a lumen.

As per Scoble (2005):

Morphologically, scales are macrotrichia, and thus homologous with the large hairs (and scales) that cover the wings of Trichoptera (caddisflies).

The Trichoptera (caddisflies) which are a sister group of the Lepidoptera have scales, but also possess caudal cerci on the abdomen, a feature absent in the Lepidoptera.

Though there is great diversity in scale form, they are structured similarly. The body or 'blade' of a typical scale consists of an upper and lower lamina. The surface of the lower lamina is smooth whereas the structure of the upper lamina is structured and intricate. Scales are attached to the substrate by a stalk or 'pedicel'.

The colouration of butterfly wings is created by the scales which are pigmented with melanins that give them blacks and browns, but blues, greens, reds and iridescence are usually created not by pigments but the microstructure of the scales. This structural coloration is the result of coherent scattering of light by the photonic crystal nature of the scales. The scales cling somewhat loosely to the wing and come off easily without harming the butterfly.

Scales play an important part in the natural history of Lepidoptera. Scales enable development of vivid or indistinct patterns which help the organism protect itself by concealment and camouflage, mimicry and warning. Besides providing insulation, dark patterns on wings provided by dark colour scales would allow sunlight to be absorbed and thus probably have a role to play in thermoregulation.Bright and distinctive colour patterns in butterflies which are distasteful to predators help communicate their aposematism (toxicity or inedibility) thus preventing a predator from preying on it. In Batesian mimicry, wing colour patterns help edible Lepidopterans mimic inedible models while in Müllerian mimicry inedible butterflies resemble each other to reduce the numbers of individuals sampled by predators.

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