About Idea iasonia (Westwood, 1848)
The Ceylon tree nymph (Idea iasonia) has a wingspan ranging from 110 to 155 mm, and is the largest member of the Danaidae butterfly family found in Sri Lanka. Both the forewings and hindwings of this species share the same pattern and colouration: overall translucent silvery-white. Forewings of the Ceylon tree nymph are almost twice as long as they are wide. Females are larger overall than males, and males have narrower forewings than females. The Ceylon tree nymph has a relatively large wing surface area compared to its body weight. This allows it to fly with very little effort and remain aloft for long periods of time. Individuals from the dry zone of Sri Lanka are typically larger and paler than individuals from the wet zone of the country. This species is endemic to Sri Lanka, and is divided into two distinct populations. The smaller, darker population occurs in Sri Lanka's wet zone, ranging from sea level up to around 1,500 m (5,000 feet). These individuals most often live in the sub-canopy of lowland tropical rain forests. The larger, lighter coloured population lives in the low country dry zone, and is usually found near water courses.
