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When resources are abundant and of the right quality, aphids tend to produce apterous forms (better equipped for taking rapid advantage of the host plant). However, faced with deteriorating local conditions, they switch to producing mainly alate forms. The greater a colony’s population, the higher the proportion of alates.

In general, in the sexual forms the females are apterous and the males alate. However, a significant number of species (10% of European aphids) show genetic variation in the male phenotype: lineages producing only alate males, others exclusively apterous males and still others generating a mixture of both types.

As resources dwindle and colony population densities rises, more and more alate morphs appear (recognisable below by the presence of wing pads characteristic of larval stage N4).
Longicaudus trirhodus	Capitophorus hippophaes	Uroleucon tanaceti	Brachycaudus populi