Scale Arrangement in the Wing: Scale Rows Around the Wing, High Scale Density Near the Leading Edge, and a Double Layer of Scales

The scale arrangement in the large region of the lepidopteran wing can be classified into two types: first, the scales form rows in the anteroposterior direction of the wing, and those anteroposterior scale rows are arranged at regular intervals in the proximodistal direction; second, the homogeneous scales are evenly or randomly distributed (see Nijhout, The development and evolution of butterfly wing pattern. Smith Institute Press, Washington, DC and London, 1991). The former is found in most macrolepidopterans, and the latter is found in many microlepidopterans and a small minority of macrolepidopterans. In both types, only near the anterior margins of the forewing, called the leading edge, the scale density increases, and consequently, the scale row is transformed into the belt of the scale group. Most of the anteroposterior scale rows and belts on the dorsal and ventral wing surfaces are continuous at both the anterior and posterior margins in the first type but only at the anterior margin in the second type. In the first type, the cover and the ground scales overlap with each other to form a virtual double layer sheet composed of scales. These two types of scale arrangements are likely related to the following functions: (i) rigidity of the leading edge in both the first and second types, (ii) generation of the leading edge vortices in both the first and second types, (iii) generation of the trailing edge vortices in the first type, (iv) scale coloring in the first type, and (v) friction reduction in the first type.