Programmed Scale Detachment from the Wing: Formation of the Highly Transparent Wing

Although most lepidopteran scales detach through friction against solid objects, they never detach from their wings only through wing flapping. However, in the pellucid hawk moth Cephonodes hylas (Sphingidae), numerous scales (d-scales) detach from a large region of the wing through wing flapping at initial take-off after eclosion. A large transparent region appears in the wing after this programmed scale detachment, while small regions along the wing margin and vein still have scales attached (a-scales). The analysis of the scale detachment process using videography revealed that the behavior of Cephonodes hylas from eclosion to initial take-off was almost the same as that of a typical hawk moth with opaque wings, which confirms that numerous scales are detached from the Cephonodes wing only through wing flapping without friction against a solid object. The morphological study of both the d- and a-scales using optical and scanning electron microscopy shows that the d-scale is obviously different in morphology and arrangement from the a-scale. Although the a-scale is common in morphology and arrangement to most lepidopteran scales, the d-scale has four distinctive features. First, the d-scale is much larger than the a-scale. Second, the d-scale pedicel, which is the slender base of the scale, is tapered, while that of the a-scale pedicel is columnar. Third, the socket on the wing surface into which the pedicel is inserted is much smaller on the d-scale than on the a-scale. Fourth, the d-scale socket density is much lower than the a-scale socket density. This novel scale morphology and arrangement of the d-scale likely helps to facilitate scale detachment through wing flapping and consequently to the appearance of a highly transparent wing region. The developmental and evolutional aspects of this programmed scale detachment are also described.