Seasonal polyphenism

Many of the tropical butterflies have distinctive seasonal forms. This phenomenon is termed seasonal polyphenism and the seasonal forms of the butterflies are called the dry-season and wet-season forms. How the season affects the genetic expression of patterns is still a subject of research.Experimental modification by ecdysone hormone treatment has demonstrated that it is possible to control the continuum of expression of variation between the wet and dry-season forms.The dry-season forms are usually more cryptic and it has been suggested that the protection offered may be an adaptation. Some also show greater dark colours in the wet-season form which may have thermoregulatory advantages by increasing ability to absorb solar radiation.

Bicyclus anynana is a species of butterfly that exhibits a clear example of seasonal polyphenism. These butterflies, endemic to Africa, have two distinct phenotypic forms that alternate according to the season. The wet-season forms have large, very apparent ventral eyespots whereas the dry-season forms have very reduced, oftentimes nonexistent, ventral eyespots. Larvae that develop in hot, wet conditions develop into wet-season adults where as those growing in the transition from the wet to the dry season, when the temperature is declining, develop into dry-season adults.This polyphenism has an adaptive role in B. anynana. In the dry-season it is disadvantageous to have conspicuous eyespots because B. anynana blend in with the brown vegetation better without eyespots. By not developing eyespots in the dry-season they can more easily camouflage themselves in the brown brush. This minimizes the risk of visually mediated predation. In the wet-season, these brown butterflies cannot as easily rely on cryptic coloration for protection because the background vegetation is green. Thus, eyespots, which may function to decrease predation, are beneficial for B. anynana to express .