Architecture of butterfly scales: Pattern formation by single cells

Abstract

Scales, the fine “dust” that covers the wings and bodies of butterflies and moths, are extremely complex in form. Like the wings and the rest of the exoskeleton they are made of non-living cuticle. Each is the product of a single cell and as such represents pattern formation on a microscopic level, all the more remarkable in view of the great variety in forms that scales may take.On the wing, the scales are usually arranged in rows of alternating long cover scales and shorter ground scales (Figure 1). The ground scales are usually unspecialized and show typical scale architecture: each is essentially a flattened sac whose lower lamina (the surface hidden from view) is smooth and featureless, while the visible upper lamina is elaborated into a gridwork of parallel longitudinal ridges joined at intervals by transverse crossribs (Figure 2). Ridges and ribs together frame rows of windows that open into the interior of the scale. The interior is usually empty except for pillar-like trabeculae which join the upper and lower laminae.