INVERTEBRATES OF THE RAINFOREST FLOOR
By Rhett Butler
| Last updated July 31, 2012
Invertebrates are by far the most abundant and most diverse animals in the rainforest. They have invaded nearly every niche imaginable and many unimaginable, and each plays a unique, although still poorly understood, role in the ecosystem. For example, in the soil invertebrates are essential in the process of decomposition. These species feed on broken-down plant plant material and organic particles. Earthworms, termites, and others fragment larger particles into sizes more manageable for bacteria, fungi, and microorganisms.
Most invertebrates in both tropical and temperate regions are small and inconspicuous but rainforests house some of the world's largest. For example, Malagasy planaria and tropical American beetles may reach six inches (15-16 cm), and centipedes may reach eight inches (20 cm). Centipedes are often brightly colored, are carnivorous, and kill their prey with poison claws located under their first body segment. In some centipede species the female carefully guards the young. Millipedes feed on rotting logs. Jewel beetles, scarabs, termites, and earthworms are part of decomposition high in the canopy, in the soil-like debris found on epiphytes. Scorpions are much smaller than their arid and temperate counterparts but often pack a powerful sting.
Leeches are fascinating rainforest dwellers even though their feeding habits repel most people. Rainforest leeches of Southeast Asia, Africa, and Madagascar, unlike the leeches of the United States, do not live in water, but are able to live terrestrially by the humid conditions of the forest. Leeches are blood-suckers that are attracted to their prey by movement, temperature, and carbon dioxide. If one sits in the rainforest of Borneo for a few minutes, leeches, moving like inch worms, can be seen approaching from the forest floor and even dropping from the trees. The victim often does not feel the bite of the leech, which has razor-sharp teeth and releases an anticoagulant into the bite to allow the blood to flow freely. Leeches are amazingly persistent, and once attached they should be removed only by dousing them with salt or shampoo or burning them with a cigarette butt. Leeches are a nuisance, but relatively harmless, since they carry no known diseases. Leeches can ingest as much as fifteen times their body weight at a single feeding, enough to satisfy them for six to twelve months before their next meal.
One of the most fearsome groups of animals in the rainforest are not jaguars, snakes, or crocodiles, but ants. Many ants in the rainforest can inflict excruciatingly painful bites and stings on the unwary forest visitor. The 24-hour ant of South American is regionally famous for its bite that can leave the victim in terrible pain for hours. However, ants also happen to be one of the most interesting and important animals in the forest as exemplified by two ant types: army ants and leafcutter ants.
Army ants of the New World have long been depicted in fictional movies and books as a marauding force that threatens everything in its path including people and entire villages. This is hardly an accurate scenario. Some rainforest peoples actually welcome the periodic visitations by army ants to clear their hut of unwanted resident pests. In addition, forest peoples have been known to use large soldier ants (also soldier leaf-cutter ants) for medical purposes. The ant is picked up by its body with its powerful mandibles open, and placed over an open wound where it is allowed to clamp the wound closed. The native then twists the head off and the jaws remain as a temporary, natural suture. Although soldier army ants are formidable with their huge jaws, the majority of ants in a given column are medium-sized worker ants. The sheer numbers of these ants enable a column to overtake animals that normal-sized colonies could not. There are reports of tethered animals being devoured, but most of the column's prey consists of other invertebrates. The column scares up many insects that usually remain hidden or camouflaged during the day. Enough of these insects are scared up to support numerous species of birds that follow ant columns and feed exclusively on the insects. Dependent on the antbirds are ithomiine butterflies which feed on their nitrogen-rich droppings. Unlike other butterflies which are restricted to food reserves built up during their caterpillar stage, these ithomiine butterflies are able to live and reproduce for months thanks to the proteins gleaned from the bird droppings. The ithomiines are safe from predation by the antbirds, because as caterpillars they feed on leaves containing poisonous alkaloids and giving the adult butterflies an unpleasant taste. Other moths and butterflies mimic the warning coloration of ithomiine butterflies to afford themselves protection.
Other species benefit from the army ant columns. Trachinid flies wait in vegetation above the ant column for grasshoppers to appear. When one does, the fly lays an egg on its body. The egg hatches into a larva which burrows into the grasshopper and devours the insect from the inside. Certain insects including wasps, beetles, and millipedes are capable of chemically mimicking the odor of army ants so they are undetectable as they move through the column, since most ants have very poor vision and can only really distinguish between light and dark. These creatures are able to get free meals in the form of prey exposed or captured by the column.
The Old World equivalents of army ants are the driver ants, which are blind and move in massive armies (20 million) under the leaf litter.
Another well-known group of ants are the leafcutter ants of the genus Atta. Large columns of these ants are a common sight in tropical regions worldwide. Foragers, are the most apparent, carrying cut pieces of leaves, petals, and fruit from their place of origin back to their nest. Leafcutter ants exist in highly structured communities in which individual size determines the ants' specialized role in the community. The largest ant type is the soldier which may weigh 300 times more than the next largest ant type, the forager. At the bottom of the size scale is a tiny ant type whose function is to ride atop the leaves as they are carried by the foragers, and keep a watch out for species of flies that lay their eggs on the leafcutters' leaves. When the larvae of this fly hatch, they can decimate the entire colony.
Leaf Cutters in Perspective
The leaves that the foragers bring back to the colony are not eaten, at least not in the conventional sense. The leaves are taken into a chamber some 15-20 feet (4.5-6 m) underground where the leaves are cut into smaller pieces by smaller worker ants. The fragments are taken to another chamber where they are chewed up by smaller ants into a leaf paste. This leaf paste provides sustenance for a certain type of fungus, which grows and is farmed by even smaller ants. This fungus is fed upon by the ants. There is no central coordination of leafcutter activities: each ant simply carries out its job based on its size and age. Communication, as for most other ants, comes in the form of chemical pheromones which cause ants to react in a certain way (though audio signals are also important in ant communication). Even the harvested plant species benefit from the work of the leaf cutters; studies have found that growth rates increase for many plant species after "pruning" by leaf cutters.
+ Pictures of floor-dwelling rainforest invertebrates
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- What are leafcutter ants?
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