Shape and molecular orientation in lepidopteran scales

Abstract

The rudiments of all types of scales on the wings of the meal moth, Ephestia sericarium (Scott) ( = E. kühniella Z.), are birefringent from the time of their emergence. Their growth may be regarded as the elongation of a cytoplasm-filled hollow cylinder of oriented protein-polysaccharide. The hair-scales grow as cylinders of approximately constant diameter, but other types of scales dilate progressively towards their distal ends, becoming club-shaped: the rate of volume increase exceeds that of surface-area increase. Dilatation is rapidly followed by flattening of the dilated region; the flattened scale continues to grow in area but undergoes little further change in shape. In spite of these transformations, the mature scale is still largely composed of oriented protein-polysaccharide, the orientation of which can be traced back in time to the earliest phase of development. Even in the mature flattened scales on the wing surfaces, sufficient orientation survives for oriented whole wings to yield a rudimentary chitin fibre-diagram when placed in the path of a beam of X-rays. Hair-scales (the ‘fur’ of moths) give an X-ray diffraction picture showing the principal reflexions of the fibre-diagram of polyacetylglucosamine. Birefringence studies suggest that the earliest rudiments contain more oriented protein than chitin, while in mature scales the chitin fraction is considerably increased. The variations in structure observed in different types of lepidopteran scales are compatible with their properties as fibrillar aggregates, and the final shapes of mature scales result from orderly displacements of the fibrillar organization laid down in the rudiment. The characteristic pattern of longitudinal ridges and transverse rungs may be compared with the patterns that arise in inorganic systems crystallizing out under certain conditions. It is suggested that the ridges compete in situ for the materials of which they are built, and that their regular spacing is an expression of this competition.