Insects were the first animals to fly. The evolutionary origin of wings remains a mystery, but we know a lot about wing formation during development from studies of insects such as Drosophila flies. The larvae of Drosophila are wingless maggots. Inside a larva are clusters of undifferentiated cells (called imaginal disks) that will form the wing. During pupation one cluster of imaginal disk cells will grow and differentiate into a wing pouch, where the wing will form. Inside the wing pouch are two rows of closely apposed epithelial cell layers that will secrete the top (dorsal) and bottom (ventral) layers of the wing membrane.
The epithelial cells secrete a cuticle (made of proteins reinforced by the polymer chitin) and other features of the wing such as hairs. At the micro-scale, the wing is not a flat surface but has ridges. The structure of the wing cuticle is controlled by the shape of the epithelial cells that secrete it. Some dorsal epithelial cells are bowed outward and the cuticle they secrete forms ridges. Rows of hairs on the top surface of the wing are precisely positioned at the apices of the ridges. Each hair is secreted at the center of an epithelial cell.
Belalcazar and colleagues* propose that the top (dorsal) surface of the wing is more rigid and the bottom surface more flexible. When the adult fly emerges from its pupa, the epithelial cells delaminate from the cuticle and retreat into the thorax. The top and bottom layers of cuticle then fuse with the bottom layer conforming to the ridges and valleys of the top layer. At genetically determined intervals, the top and bottom layers are separated to form the wing veins, channels that allow water to circulate through the wings and keep them hydrated. The micro-scale features of the insect wing are important to its ability to fly.
*Insect Wing Membrane Topography Is Determined by the Dorsal Wing Epithelium
Andrea D. Belalcazar, Kristy Doyle, Justin Hogan, David Neff, and Simon Collier. G3 January 1, 2013 vol. 3 no. 1 doi: 10.1534/g3.112.004028