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By Root 722 0

an organism has “found” or invented a successful strategy, the stage is set for replication, amplification, and modification. The great species diversity of the blues probably arose because an ancient ancestor had hit on the ant guard, thereby creating a new, safer niche for itself—one less exposed to birds and other predators as well as parasitoids. It is not likely that thousands of species of blues each independently discovered ants, but they have done different things with ants, and vice versa. Some of the most intricate and interesting associations are found in the nearly perpetual summer of the tropics.

One of the species most highly differentiated from its ancestral condition (evolved) is the moth butterfly, Liphyra brassolis, a huge Asian and Australian “blue” with a wingspread of nearly three inches. It is a relative of all other blues, but it is colored brown and black. In this butterfly the larvae make the ultimate use of ants: they are protected by living inside the nests of a very aggressive tree ant, Oecophylla smaragdina, and then they eat the ants.

These ants’ nests are made by cooperation between the young and the adults. The larvae produce silk thread from their salivary glands, and an adult holds a larva in its mandibles and waves it back and forth between two leaf edges. As the larva exudes its sticky silk, it attaches to the leaves, which become glued together to produce the shelter that is the nest—the shelter that the butterfly larvae then use as well. But how do the butterfly larvae gain entrance to the ants’ citadel? What we see now is a result of a long evolutionary arms race, a contest that these caterpillars have apparently won, because they get all the benefits whereas the ants gain nothing. Presumably, as with the spring azure in Maine, the caterpillars were originally symbiotically or at least benignly associated with ants.

The exact steps from there to here are obscure, but details of life histories suggest the problems faced and the possible solutions. At present the moth butterflies lay their eggs on the underside of branches, where they are less likely to be detected, and then the young caterpillars crawl into the leaf nest on the twig high in the tree. Undoubtedly they suffered casualties for millions of years, but over that time they evolved a thick leathery skin that ultimately became a nearly impenetrable armor and transformed the originally sluglike form of the blue’s caterpillars into a little tank. The armored caterpillars are equipped with “treads” that allow them to attach themselves to the ant’s nest substrate, primarily the leaf surfaces, so that the ants can’t turn them over and bite into their soft underbellies, and can’t detach them to throw them out.

The problem comes when the caterpillar must molt to the pupal stage, since the fresh pupal skin is necessarily soft, thin, and easily penetrable. However, these caterpillars have solved that problem, too. When they molt to the pupa they remain inside the caterpillar-tank skin, rather than discarding the skin as all other caterpillars do. But staying encased in the armor could be a problem later on, when the adult needs to emerge. To solve the problem of getting the tough armor off, the caterpillar shell has built into it predetermined lines of weakness that allow the soft, emerging butterfly to more easily crack out of the tank. However, the adult butterfly emerging within the ant nest is still necessarily soft; otherwise, it could not expand or inflate its wings, and it could then potentially be dispatched by the ants. Again, the butterfly has a special solution—unlike other butterflies, this species is covered with a dense layer of white, mothlike, powdery scales (hence the name “moth butterfly”) that gum up the mouthparts of any ants trying to bite it. The loose scales stay on long enough for the soft, still vulnerable butterfly to escape the ant nest and then inflate its wings and harden its cuticle outside the nest. The rest of the protective powdery scales are eventually sloughed off on their own.

The Oecophylla ants