Comparative scale morphology and ultrastructure across insect lineages, with special reference to curculionoids

ABSTRACT: Structuring of the cuticle underlies morphological complexity and diversity throughout insects. A common motif of cuticle formation recurs in the form of scales—structurally diverse setal modifications secreted by a single epithelial cell. Scales have evolved multiply, appearing in Zygentoma, Archaeognatha, Lepidoptera, Diptera, and Coleoptera. This study investigates the breadth of scale morphologies via SEM and TEM imaging to draw conclusions as to the patterns of convergence of these morphological features. Although studied extensively in lepidopterans, the lack of scale-less basal representatives makes it difficult to address questions pertaining to scale origins. To better understand the inception of scale development and variation derived in the process of seta-scale transformation, their evolutionary progression is here characterized in other insect groups. In particular, the curculionoid lineages are investigated to note the progression from setae or primitive scales of Nemonychidae to the elaborate, structurally diverse scales in the more derived Curculionidae. Considerations to the implications of this progression in other insect groups, including Lepidoptera, are noted.