Despite its name, the Milkweed [Butterfly - (Danaus chrysippus) is no milksop. Its other name, in fact, is closer to the truth: the Plain Tiger. This butterfly, though by no means plain, is certainly a killer.
Closely related to the North American Monarch Butterfly, but common to Saudi Arabia, the Milkweed Butterfly is a rich, honey brown color, with jet black tips on the f orewings slashed by a prominent white bar- an outfit so bright that coupled with a slow and provocative pattern of flight, it would seem to be wide open to attack from birds.
In fact though, the bright colors are a warning, and the slow provocative flight is designed to make sure that birds see the colors. Because to a bird, making a meal of a Milkweed is dangerous and the Milkweed wants the bird to know that; it's the Milkweed's chief defense. The caterpillar of the Plain Tiger, you see, lives on Milkweed plants of the family Asclepiadaceae, all of which contain some powerful poisons and the poisons are retained in the body of the adult butterfly.
Thus birds stupid enough to eat a "flying tiger" would become seriously ill - and might even die.
Birds, of course, aren't born knowing that and so, scientists think, they have to be taught by their parents. But even if they are not taught, they can learn. I have seen birds catch a Milkweed, release it immediately and then spend considerable time brushing off their beaks on the grass -showing every sign of extreme distaste.
Experiments, moreover, have shown that captive birds quickly learn to avoid eating Plain Tigers and somehow pass on the lessons to other birds. Though a few Plain Tigers get killed, therefore, they are making a sacrifice which will help the rest of their family - since the birds will never touch another Plain Tiger and will undoubtedly pass the word to their friends.
This type of defense mechanism is fascinating enough, but the story does not end there. In fact, the plot thickens. The next entry on the stage is the Diadem or Eggfly Butterfly (Hypolimnas misippus) whose popular name refers to the male, a handsome beast whose upperside is jet black with large, white, egg-shaped spots bordered by brilliant purple.
A butterfly which could not be more unlike the Plain Tiger, the Eggfly Butterfly belongs to a totally different family of butterflies which does not live on poisonous plants, and which, unlike the Plain Tiger, flies fast and furiously to escape.
But thafs the male. The female Diadem is something else: a near perfect copy of the Plain Tiger - right to the same slow, measured way of flying. Since even seasoned collectors - and birds can't tell
the difference, the female Diadem gets the same hands-off treatment as the Tiger.
This principal, known as "mimicry/' is widespread in the animal kingdom; it means that a harmless species imitates, or comes to resemble, a dangerous species to gain protection.
Obviously, one family of butterflies cannot just decide to resemble another family. The resemblance comes about by a process of natural selection taking millions of years. Eventually, though, the resemblance becomes almost perfect- and the female Diadem is safe thereafter. Even more incredibly, the Plain Tiger comes in three very distinct forms and the Diadem females have come to resemble all three.
Charles Darwin thought that mimicry in butterflies was one of the more powerful arguments in favor of his theory of evolution through natural selection. But since the concept of mimicry was first advanced -some 150 years ago - it has been the subject of considerable controversy. Despite statistical odds that seem overwhelming, other scientists claim that the resemblance between mimic and model was due to chance. And despite experimental evidence suggesting that birds do avoid both Plain Tigers and their mimics, these scientists insist that other factors are responsible.
One thing is certain, however. The argument does not bother the female Diadem. She flies around happily in southern Arabia and benefits greatly from her resemblance to the Plain Tiger. To the birds of Arabia she is truly a sheep in -well, Tiger's clothing.
Torben B. Larsen, a Danish economist, has discovered 12 new species and written numerous articles and two books about the butterflies of the Middle East.