Insect wings are flat acellular cuticular structures with rigid tubes for support. The wings of insects are formed from two layers of cells that secrete the upper and lower wing cuticle and then go away. With the cells no longer in the way, the upper and lower wing surfaces fuse together except in sclerotized (hardened) areas that remain separated and form hollow tubes called wing veins.
Wing veins perform many functions in an insects wing. The tubes deliver moisture to the wings to keep the cuticle pliable and resistant to breakage. Dried insect wings are brittle and easily shattered. Wing veins enable the wing to fold. For example, grasshopper hind wings fold like a fan. Veins support and stiffen the wing and control the wing shape when flapping.
Insects with large wings typically have more veins than insects with small wings. In insects with the smallest wings, the wing veins are often reduced in number. One vein can be adequate to deliver moisture, provide stiffness and control the wing shape. Tiny wings are less apt to become entangled in the environment, so veins for folding are not necessary. For a variety of reasons, the smallest insects typically have tiny wings with reduced venation.