If you were to see a small, fragile butterfly-caterpillar being carried off between the sharp jaws of a ferocious ant, you would probably conclude that the caterpillar was about to be dismembered and eaten by the ant - or its hungry offspring. But if that butterfly happened to belong to what is called the "Blues" family (Lycaenidae), you would be wrong.
All butterflies have a complex life cycle. The female, for example, lays up to 300 eggs, from each of which emerges a tiny caterpillar. This caterpillar does little more than eat and grow, changing its skin five or six times - as it becomes too tight - before turning into a pupa. Though this pupa is unable to move, a dramatic change immediately begins to take place inside it, and after a few weeks an adult butterfly emerges. At first its wings are tiny and limp, but within a few hours they expand to full size and harden, and the new butterfly can go off in search of food and a mate, to start the whole process again.
Normally, butterfly-caterpillars feed on the fresh leaves of green plants, and—since they can only move slowly—are subject to a wide variety of predators, including ants. But members of the Blues family have developed a special relationship with ants. (Blues, incidentally, is a bit of a misnomer; while many are blue, others come in most colors of the rainbow).
This relationship occurs because the Blues have a special gland that simply fascinates ants; located on the caterpillar's back, this gland exudes a sort of honey which the ants cannot resist when, as is normal, they begin to feed on the seeds and pests of certain green plants that the Blues also like. As soon as the ant encounters a Blue, therefore, it gets extremely excited, caresses the capterpillar with its antennae and drinks the honeydew from the gland rather than attacking and killing it.
Do the ants spare the caterpillar in return for the bribe of honey? To an extent, the answer is yes. But the relationship is actually deeper than that—as some ingenious experiments by the American entomologist, Naomi Pierce, have shown. The ants, for example, actually protect their caterpillars, driving off enemies such as parasitic wasps, assassin bugs and other predators. And some caterpillars actually need the attention of ants: if the honey is not removed, a mildew develops which can kill the caterpillar. Thus the ants get honey, the caterpillar gets protection and both parties benefit in a perfect example of what is called "commensalism"—a scientific term for, "I'll scratch your back if you scratch mine".
While unusual, commensalism is not unique. It exists also between ants and greenflies (aphids), which also provide ants with a form of honey. And in Arabia, caterpillars of the African Emigrant Butterfly (Catopsilia florella) forage on the same plant as ants—a species of Cassia, from the pea family—and also provide a sugary substance to ants in return for a truce.
What is unique is the way in which the Blues have gone a step further with these arrangements: some Blues actually live inside ants' nests, are fed by the ants and even eat immature ants (eggs, grubs, and pupae). A few have even specialized in living inside the nest of the particularly savage Tailor Ant (Oecophylla).
Five such species live in Arabia. The two Leopard Butterflies (Apharitis) are the best examples. Rare creatures found in tight little colonies, their females lay eggs near the nests of the host ant, and, as soon as the eggs hatch, the ants carry the tiny caterpillars inside their nest. Sometimes the ants' nest is underground, but in some parts of Arabia the butterflies seem to prefer ants which live inside the trunks of old date palms.
Initially, the caterpillars are fed by the ants, but as they grow larger they start to use their own initiative, eating ant eggs, grubs and pupae. Curiously, the ants let them do this without attacking them—partly because the ants continue to want the honey, but also because the caterpillar can probably synthesize the very special scents by which the ants recognize each other. A sort of I.D. card, these scents (pheronomes) are unique to each ant nest, and without them ants from one nest, even of the same species, will be instantly attacked by ants from another nest. Similarly, if the caterpillar of the Leopard Butterfly were switched to another nest, the ants there would attack it fiercely.
Once the caterpillar is fully grown, it pupates near the exit holes of the nest and, some weeks later, crawls out, dries its wings and starts to look for a mate. Since it doesn't need to search for food at the same time—it emerges from the pupa with plenty of stored fat—the Leopard Butterflies do not fly far from their birth place, a useful adaptation since it might be difficult to find another suitable nest.
The ability to live underground with ants and to carry food reserves is very useful; as one result, these butterflies do not have to seek flowers to sip nectar from—as most other kinds do—and this allows one species to live permanently in the deserts of the Empty Quarter (Rub' al-Khali)—where they were found by both Harry St. John Philby and Bertram Thomas in the 1930's. No other butterflies, in fact, can survive under such conditions.
The Giant Cupids (Lepidochrysops) of southwestern Arabia have a somewhat different relationship with ants. Their females lay eggs on plants of the mint family and the newly emerged caterpillars eat the tender leaves and flower buds of their host plant, like normal butterfly-caterpillars. At first they are wonderfully camouflaged, but at the third change of skin white caterpillars with hunched shoulders emerge and immediately drop to the ground where they are picked up by ants and taken off to a nest.
From then on the Cupids are set for life. They will, thereafter, feed on the immature stages of their host, giving honey in return, just like the Leopard Butterflies.
Altogether, there are three species of Giant Cupids in Arabia (two of them recently discovered by the author), and several hundred in Africa (where the three Arabian species are not found); the reason Cupids are so prolific is that by the time they find the right ants—ants that will protect them and not devour them—they will also have found the right food supply and the right ecological conditions. There is a penalty, however; the right combination of circumstances is rare and, as a result, most Cupids live in small, widely dispersed colonies.
To observers, the ants may seem to be getting an unfair deal from Giant Cupids and Leopard Butterflies—since the butterflies, which live off young ants, can't exist without the ant, while the ants can live perfectly well without the butterflies. All the ants get, it seems, is some honey. But then observers, perhaps, have never tried caterpillar honey.
Torben B. Larsen writes regularly for Aramco World on the entomology of the Arabian Peninsula.