Blue Moon

Male blue moon butterfly Hypolimnas bolina, photographed by Comacontrol.

My native country of New Zealand is not home to a large diversity of butterflies. Only a couple of dozen or so species are known from the entire country. It would not be unreasonable for a keen butterfly spotter to attempt to track down them all. But one particular species of butterfly generally included in New Zealand lists would a touch of luck: the aptly named blue moon Hypolimnas bolina.

This is because the blue moon is not a regular resident of New Zealand (I've personally never spotted one). It is native to a wide region stretching from Madagascar and India to Japan and northern Australasia where it is usually referred to by the more prosaic name of common or greater eggfly. The only examples found in New Zealand are vagrants who lost their way on southwards migrations. Nevertheless, such vagrants are regular enough for its local appellation to be thought worth coining. Not only does it reflect their rarity, it also describes the appearance of the male, with the wings bearing blue-ringed white spots on a black background.

Two females of Hypolimnas bolina. On the left, a mimetic individual, copyright Greg Hume; on the right, a non-mimetic individual, copyright W. A. Djatmiko.

The appearance of the female is a bit harder to explain because it can vary between individuals. Females of the eggfly genus Hypolimnas are commonly mimics of other, poisonous butterflies of the subfamily Danainae, to which eggflies are only distantly related (both groups belong to the family Nymphalidae but eggflies are placed in the subfamily Nymphalinae). For instance, the diadem or danaid eggfly H. misippus of Africa and Asia (and also introduced into parts of the Americas adjoining the Caribbean) is a mimic of the plain tiger Danaus chrysippus. The chosen model of H. bolina in the western part of its range is the common crow Euploea core and in the region of India almost all females are a remarkably good copy of that species (above left). But as one moves east, one starts seeing females of H. bolina that are not mimics like the individual shown above right; by the time one reaches Australia these make up the greater part of the population. Mimetic females may also vary to resemble different Euploea species, depending on which model is locally present.

Female danaid eggfly Hypolimnas misippus, copyright Raju Kasambe. Males of this species are similar to those of H. bolina but lack the blue rings around the white spots on the wings.
There are about two dozen species of Hypolimnas eggflies found in various parts of the Old World tropics. Hypolimnas misippus is also found in parts of the Americas around the Caribbean where its presence is usually explained as the result of an early introduction (possibly, and somewhat poignantly, in connection to the slave trade). Their vernacular name is probably derived from the unique behaviour (for butterflies) of a number of species whose females stand guard over their eggs, beating their wings over them to protect them from predators until hatching. About two-thirds of Hypolimnas species are mimics. In some of these species, both sexes are mimetic; others resemble H. bolina and H. misippus in that only the females are mimics (Vane-Wright et al. 1977). One might be tempted to ask why this variation exists. One point to be considered is that there are limits on when mimicry is likely to be effective. The mimic needs to be much less abundant than its model, otherwise potential predators may not learn to associate the distinctive coloration with the toxic original. Swinhoe (1896) noted that males of Hypolimnas misippus were very active, aggressively defending their territories from other butterflies, and suggested that this agility might provide males with alternative defences to mimicry. The more sedentary females (especially when egg-guarding) might be expected to benefit more from the passive protection mimicry provides, but mimesis might be expected to disappear in areas where their model is less abundant.

REFERENCES

Swinhoe, C. 1896. On mimicry in butterflies of the genus Hypolimnas. Journal of the Linnean Society, Zoology 25: 339–348.

Vane-Wright, R. I., P. R. Ackery & R. L. Smiles. 1977. The polymorphism, mimicry, and host plant relationships of Hypolimnas butterflies. Zoological Journal of the Linnean Society 9: 285–297.