Branched Filamentary Scales Along the Wing Margin of the Small Moth: Coherent Scale Array in Flight and Compact Scale Bundle at Rest

In most lepidopterans (butterflies and moths), the wing margins are fringed with specialized scales, called marginal scales. Most of them are slenderer than common scales which cover almost all wing regions. In the hindwings of some small moths, the posterior margins are fringed with particularly long filamentary scales. Despite the small size of these moths, their marginal scales are much longer than those of large lepidopterans. This chapter addresses the long filamentary marginal scales of a small moth, Phthorimaea operculella (Gelechiidae). The morphological features of those scales are as follows. First, a wide array of long filamentary scales maintains a coherent sheet-like structure during the wingbeat. Second, each filamentary scale consists of a proximal stalk and a distal branch; the stalk length varies from scale to scale, and the branching number of each scale is from two to six. Third, the microstructure of the filamentary scale is almost the same as that of the common scale. Fourth, the scales with stalks of various lengths are distributed along the wing margin. Fifth, filamentary scales of various lengths coexist along any small section of the wing margin, which implies that the scale branches are ubiquitously and densely distributed within the wide scale array to form a mesh-like structure similar to a nonwoven fabric. Based on these morphological features of the branched filamentary scales, it is proposed that the coherent scale array during the wingbeat is maintained through the ubiquitous interactions among the branched filamentary scales and that the coherence of the wide-scale array likely contributes to efficient flight performance. Furthermore, it is also proposed that the Phthorimaea hind wing, which is composed of a wide scale array and a narrow membranous region, likely contributes to the formation of a compact posture at rest, which decreases the wind power applied to the moth.