1. Winged morph production in aphids is a phenotypic trait that has traditionally been seen as a response to unfavourable environmental conditions. The evidence to support this theory is reviewed and the ecological and evolutionary significance of the findings is discussed.
2. The common assertion of poor host-plant nutritional quality leading to increased production of winged morphs does not always apply, particularly when the host-plant quality is exceptionally poor. The available data are skewed heavily towards Myzus persicae, and for this species the dynamical change in plant quality appears to be important with respect to wing induction.
3. Crowding may be a less influential stimulus for wing induction as study methods approach natural conditions experienced by aphids on their host plant.
4. The growing evidence that interactions with other organisms can induce the production of winged morphs by aphid colonies is reviewed. In the case of natural enemies, such a response by an aphid colony may be regarded as induced defence. Wing induction may also act as a means of transmission for a virus or fungal pathogen.