Insect epithelial cells contain cellular extensions such as bristles, hairs, and scales. These cellular extensions are homologous structures that differ in morphology and function. They contain actin bundles that dictate their cellular morphology. While the organization, function, and identity of the major actin-bundling proteins in bristles and hairs are known, this information on scales is unknown. In this study, we characterized the development of scales and the role of actin bundles in the mosquito, Aedes aegypti. We show that scales undergo drastic morphological changes during development, from a cylindrical to flat shape with longer membrane invagination. Scale actin-bundle distribution changes from the symmetrical organization of actin bundles located throughout the bristle membrane to an asymmetrical organization. By chemically inhibiting actin polymerization and by knocking out the forked gene in the mosquito (Ae-Forked; a known actin-bundling protein) by CRISPR-Cas9 gene editing, we showed that actin bundles are required for shaping bristle, hair, and scale morphology. We demonstrated that actin bundles and Ae-Forked are required for bristle elongation, but not for that of scales. In scales, actin bundles are required for width formation. In summary, our results reveal, for the first time, the developmental process of mosquito scale formation and also the role of actin bundles and actin-bundle proteins in scale morphogenesis. Moreover, our results reveal that although scale and bristle are thought to be homologous structures, actin bundles have a differential requirement in shaping mosquito scales compared to bristles.

Insect epithelial cells contain unique cellular extensions such as bristles, hairs, and scales. In contrast to bristle and hair, which are not divergent in their shape, scale morphology shows high diversity. In our attempt to characterize the role of the insect‐specific gene, Spindle‐F (spn‐F), in mosquito development, we revealed a scale‐type specific requirement for the mosquito Aedes aegypti spn‐F homologue. Using CRISPR‐Cas9, we generated Ae‐spn‐F mutants and found that Ae‐spn‐F is an essential gene, but we were able to recover a few adult escapers. These escapers could not fly nor move, and died after 3 to 4 days. We found that in Ae‐spn‐F mutants, only the tip part of the bristle was affected with bulbous with misoriented ribs. We also show that in Ae‐spn‐F mutants, only in falcate scales, which are curved with a sharp or narrowly rounded apex, and not in other scale types, the tip region is strongly affected. Our analysis also revealed that in contrast to Drosophila spn‐F, which show strong defects in both the actin and microtubule (MT) network in the bristle, the Ae‐spn‐F gene is required only for MT organization in scales and bristles. In summary, our results reveal that Ae‐spn‐F is required for shaping tapered epithelial cellular extension structures, namely, the bristle and falcate scales by affecting MT organization.