The Union Jack butterfly is found in various altitudes, from sea level to up to 7,874 feet. Like some other butterfly species living in the Australian rainforest, the Union Jack is found in the canopy region, as well as at ground levels and at the edges. This species rarely migrates from one habitat to the other.
The Union Jack butterfly has a wingspan of about 2.5 inches. Males and females vary slightly in color and pattern arrangement. However, Union Jack butterflies generally have predominantly white wings with black borders. The undersides of the wings feature yellow to orange and red markings in both sexes of most subspecies. The upper side of the wings is often darker in females.
Part of the family Pieridae (whites and sulphurs), the Union Jack butterfly has several known subspecies, which show slight variations in color and pattern. In Australia, Delias mysis mysis is more common in Queensland, while Delias mysis aestiva is abundant in the Northern Territory. Other subspecies include Delias mysis lara and Delias mysis onca, common in some areas of Papua New Guinea, Delias mysis rosselliana, found in some islands of the east coast of Australia, and Delias mysis maforensis, found in Indonesia.
Unlike many species of butterflies which do not feed at all, Union Jack adults eat the nectar of several tropical species. However, they prefer the dense and bright-red cluster flowers of the genus Callistemon, common in Australia and neighboring islands. During the larval stage, Union Jack caterpillars feed exclusively on leaves, preferring the plants of the order Santalales or mistletoes.
Like other butterflies species, the Union Jack undergoes four development stages during its life cycle. After mating, females lay clusters of eggs on leaves of several species of mistletoe, especially the smooth mistletoe (Dendrophthoe glabrescens). Caterpillars have black heads, yellowish green bodies and long white hairs. The pupa is often yellow, and found attached to the bottom of leaves.