• Choose a category
• All
• Classic-Fiction

By Root 566 0

or edges to orient themselves upward. When a worker reaches a horizontal leaf splay, she tends to stay on its upper surface, where she is less likely to be knocked off. She can survey a leaf by moving along its edge to trace its outline, deviating if she chooses to explore the leaf’s center or underside before going back to the margin. When she departs a leaf and encounters a branching point among the twigs, her best bet is to always turn onto the next stem in a consistent direction—say, to her left. By tracing leaf outlines and being consistent in her choices at forks, she is able to move through a leaf spray without revisiting the same leaf twice and without ever having to mark a route or memorize the terrain.12 This technique enables an ant to examine a plant more efficiently than is possible when she explores the ground. That is true even when the ground is flat and bare, which is rare: ground-dwelling ants navigate through jumbled decaying matter and plant parts, geometries far more haphazard than trees. For weaver ants, the solution is to commute on the ground as they do in the canopy, following crestlines offered by exposed roots or fallen sticks.

Once an ant begins to range widely from one tree to the next, the messiness of canopy topography forces her to rely on a variety of orientation cues. Leafcutter ants, for example, measure thermal radiation to locate the sun-warmed foliage they prefer to cut.13 Visual cues may also be valuable. A weaver ant can choose a course at a particular angle relative to the sun or moon; if the sky is shaded, she uses a less accurate, internal magnetic compass.14 Some ants create maps by taking mental snapshots of the greenery against the sky.15 Although it may be relatively easy to use these snapshots to navigate on flat ground, using them within the trees must require an overwhelming feat of insect memory.

A good memory can be essential to canopy survival. The weaver ants Ed Wilson raised on a small citrus tree in his office when I was his graduate student became excited when a novel object such as another tree was placed nearby, gathering on a twig in an attempt to reach it. Apparently they remembered enough of their surroundings to recognize this change. We might expect the workers in a large colony to keep to the small portion of the territory that they know well, and in fact weaver ants on border patrol don’t move around a lot.16

Age and experience can play a big role when ants explore farther afield. The chemical trails of bullet ants, for example, often cross one another among the interdigitating branches and vines of the canopy, and the workers can distinguish the scent routes laid by different nestmates to different destinations.17 These ants also have exceptionally good eyesight and keep track of their whereabouts by eventually memorizing the location of such landmarks as branches.18 Novice workers follow the trails, while their more experienced compatriots come to navigate by the landmarks almost exclusively.19

In Queensland, weaver ants—known in Australia as green tree ants because of their coloration—form a chain to connect branches during a foraging expedition. In most situations the chain is many ants thick.

It’s not easy even for arboreal species to range beyond one tree. Rainforest crowns are separated by open spaces and seldom intermix. Vines offer shortcuts as well as an abundance of honeydew-producing insects, explaining why weaver ants prosper at forest margins where the canopy is cluttered with such connections. Adjacent tall trees lacking vines tend not to be occupied by the same weaver ant colony. However, in these situations weaver ants can create their own shortcuts. I saw this ten years ago during a stay on the north coast of Papua New Guinea. I was drying myself off after snorkeling when I noticed, stretched between the branches of two citrus trees a few meters above my head, a chain of weaver ants 6 centimeters long. I broke the chain with a finger to see what would happen. Ants accumulated at the site, climbing on one another in the direction of