Adventures Among Ants - Mark W. Moffett [81]
Although plant sap is low in nutrients, it offers energy and sufficient nourishment, provided the ant drinks enough of it. The most successful canopy ants are therefore built to tank up.33 The workers of some species transport droplets between their jaws to other ants, which drink the sap as if from a bucket. Other species carry the liquid in a thin-walled internal sac called a crop, from which they take food into their stomachs as needed or regurgitate it for their sisters. This makes a convenient storage place, and it leaves the mandibles free for other work.34 However it’s done, fluid meals are transferred from ant to ant so that each receives a sampling of the nutrients passing through the colony at the time and is aware of the colony’s general food needs.35 The ant superorganism has, in effect, a social stomach—an approach that even Napoleon, whose army traveled on its stomach, never imagined.
Nutrients aren’t everything, however; that stomach needs to hydrate, too, and drinking water can be in short supply. I discovered that the nest entrances of a certain south Indian ant resemble dead birds because the workers decorate them with feathers, which collect dew each morning—a sort of proto-tool for harvesting moisture.36 Even in a wet rainforest, rain quickly drips out of trees, leaving the canopy parched much of the time. For weaver ants and other species, sap is a prime water source. Workers often drink from wounds on vegetation. Typically, they prefer the watery sap from leaves and twigs over the unpleasantly viscous fluid from the bark that prevents infection of the trunk and can harden and fossilize as amber. (Ants can get caught in this sap, which is bad for them but sometimes useful for us. In one deposit in Kenya, a population of thirty-million-year-old weavers, preserved in crystalline form, revealed that the division of labor, based on similar minor and major workers, was probably organized then much as it is today.)37
When it comes to nutrition, though, much more desirable than everyday plant sap are the enriched, sweet fluids secreted by glands on the plant surface. Even more than the ground-strewn bonanzas of fruit and meat frequented by the marauder ant, these nectaries tend to be both persistent and distributed in patches.
Traveling around tropical Asia, I noticed blemishes on the leaves of many dipterocarps, the local tree giants. Later I learned that these “green spots” exude nectar.38 Unlike the nectar-producing organs inside flowers, which attract bees, butterflies, and other pollinators, so-called extrafloral nectaries like the green spots occur on leaves and stems of diverse plants and are tailored to ant cravings. Oecophylla are among the many ant species that guard such nectaries, and in so doing they protect the plants by making lunch out of any foliage-eating insects nearby. Some nectaries even develop close to flowers or fruit, thereby ensuring protection for the plant’s reproductive parts during its breeding season.39
Weaver ants don’t just seek out food sources such as fruit flies, bats, and green spots. They also, fascinatingly, farm them—in the form of such insects as mealybugs, scale insects, and plant hoppers, which many tropical ants care for the way ants tend aphids in the temperate zones.40 Classified by biologists as Homoptera, these sap-sucking species excrete excess plant fluids in a condensed form known as honeydew—often directly into the mouths of ants.41 Their excrement, which is more nutritious than nectar, is considered delectable even by some humans: in the Bible, it’s called manna.
Weaver ants tend many kinds of Homoptera, as well as certain caterpillars that produce similar sweet secretions. These “cattle” range from species that do fine without the ants to a few that are found only where weaver ants thrive. The ants are as protective of their livestock as any cowboy is