The Ceylon tree nymph (Idea iasonia) is a beautiful silvery white butterfly (2) and is the largest member of the Danaidae family in Sri Lanka (3). Both wings are a translucent silvery white, and the forewings are almost twice as long as they are wide (3). The sexes are similar, although females tend to be larger in size (2) and males have narrower wings (3). Individuals from the dry zone tend to be larger and lighter in colour than those from the wet zone, and some individuals may have a reddish hue over the wing surface (3). The fact that the surface area of the wings is very large relative to the weight of this butterfly allows it to fly with the greatest of ease. It can stay aloft for very long periods of time (2), with such slow, effortless wing beats that the sequence of wing movements is easy to observe (3).
Although the Ceylon tree nymph spends much of its time flying leisurely high in the tree canopy, it does descend to the ground to feed on nectar and to breed (3). During courtship, males and females fly together for a long period and the male extends two brush-like organs (known as hair pencils) from the tip of his abdomen. These hair pencils brush against the female’s antennae and release pheromones and other chemicals that stimulate her to mate (4). One of these chemicals, danaidone, is poisonous, and is also passed to the female with the sperm during mating. This substance is gathered by the adult males from various plants and deters predators from eating the butterflies. Some of the danaidone passed to the female from the male during mating ends up in the tissues of the female and in her eggs, protecting both the female and the eggs from predation (4).
This species is classified as Near Threatened by the IUCN Red List. As yet, direct conservation action has not been targeted at this species, although it does occur in a number of protected sites, including the forest reserves of Kanneliya, Sinharaja and Morapitiya (5)(6).
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