Madagascan Sunset Moth

Appearance

"Chrysiridia rhipheus" has a wingspan of 7–9 centimetres, and sometimes up to 11 centimetres. Moths from the highlands, 900–1,080 metres, have a median wingspan of 7 cm; moths from lower altitudes, 600 m, have a median wingspan of 9 cm. Like many other uraniine moths, the sunset moth has an uncanny resemblance to swallowtail butterflies, especially in its tails and colourful wings, and can easily be mistaken for a butterfly.

The sunset moth is black with iridescent red, blue and green markings. There is a fringe of white scales on the wing edges, wider on the hindwings. The moth has six tails, very often lost or damaged during its life. Pattern variations are common, and the moth is often partly asymmetrical; one of the factors causing this is temperature shock during the pupal stage.

Distribution

Drury's specimen was given to him by Captain May of Hammersmith and believed to have come from China. Cramer believed the specimen came from Chandernagore in Bengal, however, giving rise to the French name "page de Chadernagor". It is now known that "Chrysiridia rhipheus" is endemic to Madagascar. Thousands of adult moths seasonally migrate between geographically isolated populations of their host plant "Omphalea" spp. They can be found almost everywhere on the island, except in the south-west and the extreme subdesertic south of the Androy where their host plant is absent. They migrate from the three species in the dry deciduous forest in the west to the eastern rainforest species. The western species are largely in protected areas. The eastern species, on the other hand, is mostly unprotected and dispersed in widely scattered populations threatened by deforestation. Being the only evergreen species, "O. oppositifolia" is probably crucial for the moth's survival. The Jamaican moth "Urania sloanus", from the same subfamily, most likely became extinct after the loss of one of its host plant species.

The moths migrate in response to changes in the host plants. "Chrysiridia" larvae defoliate the whole plant, and even eat the flowers and fruit, and thus have a considerable negative impact on the reproduction and survival of seedlings. The plants probably react by changing their nutrient and secondary compound levels, becoming toxic to the larvae and causing high mortality. "Omphalea" populations that are not damaged by moths for long periods of time have lower toxicity. These factors cause mass increases in local population, followed by sudden crashes. The population crashes might result from increased larval mortality, but are more likely caused by the emigration of the adult moths. Through semiochemicals, the plant may recruit hymenopteran parasitoids as a protection, hence playing a role in the population dynamics of the moth.

Behavior

Unlike most moths, the sunset moth is day flying and the bright aposematic colours warn predators of its toxicity, a strategy seen in many diurnal moths.

Another habit the moth has in common with many butterflies is the night resting posture—the wings are held vertically over the back. During their migrations they roost in a group for the night. The moth flies over the crown of trees and in clearings. Sometimes because of high winds it is carried about 100 m, it then falls, inert and wings closed, to the ground.Continuous generations of the moth are present all year. The highest populations are found from March to August, while the lowest are from October to December.
The females lay their eggs late in the afternoon or at nightfall, and locate potential oviposition sites visually. Like the eggs of other Uraniidae, the sunset moth's eggs are domed with projecting ribs. A single egg weighs about 1 milligram and usually has 17 ribs, but sometimes 18 or less often 16. The eggs are usually laid on the lower surface of "Omphalea" leaves, but occasionally on the upper surface. Eggs are laid in groups of 60 to 110, usually about 80.

After they hatch, the small caterpillars only eat the tissue between the veins of the leaves. They do this to avoid the sticky and toxic latex produced by the plant's laticifers and transported in the veins. After 3–4 days, the caterpillars also eat flowers, fruit, tendrils, petioles and young stems, defoliating the entire plant. They are particularly fond of the glands at the base of the leaf, near the petiole. They can deal with the chemical defences in the latex, which does not cause the problem of mouthpart coagulation. The caterpillars spin silk from their mouth with an '∞' motion of the head as they walk, this keeps them from falling from the smooth surface of the leaves. The silk also permits them to climb back to the plant should they fall. Strong rain makes them fall despite the silk. There are four instars, and the caterpillar stage lasts from two months in the warm season to two and a half to three months in the cold season. The caterpillar is whitish yellow with black spots and red feet and is covered in club-ended black setae. It has five pairs of prolegs on segments 3 to 6 and 10, and six true legs attached to the thorax.

After completing all but its last moult, the caterpillar spins a cocoon out of silk. The cocoon can be in the tree crown or between two leaves, but is most often near the ground, between moss and bark. It is an open network cocoon with large and irregular mesh. In the warm season, the cocoon takes about 10 hours to spin, the metamorphosis takes place about 29 hours later and lasts about 6 minutes. These durations are slightly longer in the cold season. The chrysalis stage lasts 17 days in November, the warmest month, and 23 days in July, the coldest month. Five to six days before eclosion, the motifs of the wings start to become visible. The moth emerges during the night or in low light, by splitting the pupal case from the top. Once out of the pupal exuvia, the moth finds a horizontal surface, from which it suspends itself by its four anterior legs. The wings are deployed in about 10 minutes, by pumping haemolymph into the wing veins. The moth then beats them a few times, waits 45 minutes to let them harden, then beats them lightly again. The moth finally takes flight between one-and-a-half and two hours later.