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The wings of ostriches, emus and other flightless birds are vestigial; they are remnants of their flying ancestors' wings. These birds go through the effort of developing wings, even though most birds are too large to use the wings successfully.
Brachyptery is an anatomical condition in which an animal has very reduced wings. Such animals or their wings may be described as "brachypterous". Another descriptor for very small wings is microptery. Brachypterous wings generally are not functional as organs of flight and often seem to be totally functionless and vestigial.
Wing structure and colouration often vary with morphs, such as in the aphids, migratory phases of locusts and polymorphic butterflies. At rest, the wings may be held flat, or folded a number of times along specific patterns; most typically, it is the hindwings which are folded, but in a few groups such as the vespid wasps, it is the forewings.
The vestigial gene is considered to be one of the most important genes for wing formation, but when it becomes over expressed the issue of ectopic wings begin to form. [71] The vestigial gene acts to regulate the expression of the wing imaginal discs in the embryo and acts with other genes to regulate the development of the wings.
The vestigial wings are so small that they are invisible under the bristly, hair-like, two-branched feathers. While most adult birds have bones with hollow insides to minimise weight and make flight practicable, kiwi have marrow , like mammals and the young of other birds.
[19] [20] Some very small insects use the fling-and-clap or Weis-Fogh mechanism in which the wings clap together above the insect's body and then fling apart. As they fling open, the air gets sucked in and creates a vortex over each wing. This bound vortex then moves across the wing and, in the clap, acts as the starting vortex for the other wing.
The majority of insects have two pairs of wings. Flies possess only one set of lift-generating wings and one set of halteres. The order name for flies, "Diptera", literally means "two wings", but there is another order of insect which has evolved flight with only two wings: strepsipterans, or stylops; [2] they are the only other organisms that possess two wings and two halteres. [6]
Several Dpp target genes which have been identified are spalt-related, vestigial, Serum Response Factor, and achaete-scute. [4] Ubx also represses Wingless in the posterior compartment of the dorsoventral axis. The products of these genes are used in the regulation of morphological features between the wing and the haltere.