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that host these caterpillars are very aggressive, and so they are all the more useful to any caterpillars that can breach the ants’ defenses. These day-active ants are also aggressive toward other ant species and try to evict them from their trees (kill them) whenever possible. Yet another ant species, Polyrhachis queenslandica, may live on the same tree with the Oecophylla. It avoids overlapping by being strictly nocturnal. In the daytime, when the Oecophylla are active, the Polyrhachis avoid being killed by staying in their nests—a couple of superimposed leaves glued together and sealed along the sides, with only two narrow tubelike entrances built into opposite ends of the nest. During the daytime, guard ants position themselves at these entrances and neatly plug them with their flat heads. No Oecophylla can get past these head plugs to enter, nor can any caterpillars. Nevertheless, these ants also host a blue caterpillar—one that has evolved a strategy exactly opposite that of the moth butterfly.

This blue, Arhopala wildei, lays its eggs directly on the Polyrhachis queenslandica ant nests or on twigs near one of the two nest entrances. To these ants, the caterpillars are not enemies; but their Oecophylla enemies on the same tree would eat them. To avoid that fate, these blues’ eggs hatch at night when the Oecophylla sleep, and the nocturnal Polyrhachis ants then safely carry the young caterpillars into their nest before dawn. Once inside they treat these blues’ caterpillars as their own larvae, and when the caterpillars molt the ants even assist them by pulling apart the cuticle to help them emerge, in the same way that they help their own larvae to molt. And if the nest is disturbed the ants carry the caterpillars to safety along with their own brood. The bribe? The A. wildei caterpillars have a gland on their posterior end that produces an ambrosial (to the ants) exudate. The caterpillars offer this tasty treat by raising their rear end to allow direct access to it whenever an ant approaches them. Apparently they have a scent that mimics that of the ant larvae so that the caterpillar is confused with one of them. Meanwhile, the treat ultimately comes from the ants themselves, because the caterpillars gorge themselves on the ants’ eggs, larvae, and pupae.

There is so far no explanation why these blues’ life cycles are so much more complex than that of the familiar azure that ushers in the short summer in Maine. But perhaps in a perpetual summer, such as a tropical one, there is more time and opportunity to evolve complex social relationships.

By far the majority of the larvae of the world’s butterflies and moths feed on leaves, often the plentifully available leaves of trees, and one might not expect that much sophistication and intrigue might be involved in the behavior of harvesting them. But, as the blues indicate, one should never underestimate even a caterpillar!

8

Artful Diners

2 August 2006. TEMPERATURES ARE NOW REGULARLY IN the high eighties and low nineties, and as usual, the air is muggy and sweltering. The cicadas’ buzz is now the definitive summer sound. The house wrens are totally silent—a big change! The eggs of this clutch (their last) are about to hatch. Do they already know this? Is that why the male now no longer sings? In the trees I see a wiggle of leaves here and there, as birds search for caterpillars. I saw the first roadkills of the large caterpillars—mostly sphingids—about ten days ago; they are leaving their food trees to wander before pupating underground. Monarchs float by lazily, but they occasionally accelerate in a few wing beats. One of them flies in a more steady, stately manner, and then lands on a bush. It spreads out its wings, and I notice that another one (which has its wings closed) dangles from it, attached by its genitals. Extreme mate-guarding. Maybe I will soon see more of their white, yellow, black-striped caterpillars on the milkweeds. Viceroy butterflies, the monarch’s mimics, are making their first appearance. At night we see the flashes of distant lightning