Of those that are wingless, some are primitively wingless and others are secondarily wingless . Primitively wingless insects are those which never evolved wings and are typically very primitive insects. Wing number, form and venation are important diagnostic tools. There are four major wing types. The most common wing form is membranous with a system of veins which give strength and rigidity.
Membranous wings are thin and more or less transparent.
Hemelytra are wings which are part membranous and part leathery, as seen in the front wings of stink bugs and other true bugs. Tegmina are the leathery front wings of grasshoppers, crickets, cockroaches and their allies. Elytra are the thickened, leathery or horny front wings generally found in beetles. A fifth structure, the haltere , is a highly modified wing found on the metathorax of true flies. It is knobbed and represents the hind wings which have been modified into balancing organs.