Parasol ants

Zoological parks exhibit mostly vertebrates, which in general are larger animals. But of late they have been trying to include invertebrates too among their exhibits. In 1984, during the 17th International Congress of Entomology at Hamburg, we visited the zoo there, Carl Hagenbecks Tierpark, where we saw a remarkable exhibit of parasol ants. In a glass covered enclosure a log, looking like a limb of a tree, was placed nearly horizontally. At one end of the log was an earth mound, enclosing an ant nest, and at the other end were twigs with fresh leaves. Ants were seen swiftly moving on the log towards the leaves, and there was another stream of ants moving on the log from the leafy branches towards the nest. Movements in the latter group were cautious and disciplined. Ants in this stream were moving in a single file, each ant carrying a cut away piece of leaf, caught between her mandibles, swaying over her head and looking like a parasol. The serene and orderly procession of seemingly parasol carrying little ladies was an interesting and a comical sight. At the Museum of Natural History, in Geneva, in the same period was a similar exhibit, and the ants were "fed" with bramble (Rubus) leaves, available all the year round in that area. In the streets of Brasilia, you see them along the sidewalks, going their way and transporting their leaves or bits of flowers, while the street is full of passing cars. In the Vigosa University, in Brazil, they used to "feed" them with Hibiscus flowers, an Asiatic plant which they specially like. Normally they don't feed on Cecropia trees, inhabited by the ants A%teca, and there are other exceptions, and certain trees, the myrmecophobic ones, which are toxic, are excluded in their menu. PJ has seen that in Panama an Acacia tree was defoliated in less than one day by the Atta ants, which are parasol ants. When some trees have been defoliated in an area, they attack different plants the next day. Those Atta seem to be ecologically minded. Some genera of these ants are specialized for feeding on flowers and grasses, which they prefer to trees.

On returning, the ants enter the nest with their leafy trophy. But they are not leaf feeders. They use the leaf pieces for growing a fungus, and they are fungus feeders. There are some chambers in the nest, which are large and are used for developing fungus gardens, and the leaf pieces provide an organic base for growing the fungus. The relationship between fungi and ants have been recently extensively studied. The discovery of fungus feeding is due to Belt, an engineer, in Nicaragua during the XIX century.

Leaf-cutter ants live exclusively in the tropical or subtropical parts of the New World. In the Old world, termites fill their niche and cultivate fungi. Fungi, associated with ants, decompose cellulose of the plant tissues for them, and termites manage that with the help of endosymbiotic Protozoa in their intestine.

New colonies of parasol ants are started in much the same way as in case of other ants. Occasionally winged queen ants are produced in the colony. A queen ant, which has freshly emerged from the pupal skin, makes a pellet of fungal filaments or mycelia in a fungal garden, puts the pellet in a small pocket at the base of her mouth parts, and flies away. After mating with a winged male during her flight, she settles down at a suitable spot, sheds her wings, and makes a small chamber in soil, which is the beginning of a new nest. She lays a large number of eggs, deposits her excreta and places over it the fungal pellet. The excreta provide the first fertilizer for the fungal growth.

The female, laying the foundations of a new colony, is markedly ovivorous, that is she eats most of her own eggs (Poole et al, 1963). The main reason for this is that she has to produce more excreta for growing the fungal mass, and at this stage worker ants are not available for bringing in leaf pieces for the fungus garden in making. As soon as the first batch of workers have completed their development, collection of leaf pieces will start, and the mother queen will confine her activity to egg laying.

While larger workers collect leaf pieces, smaller workers, the minimes, remain within the nest, and, receiving those pieces, they bite them into a pulp, adding their saliva to it. The pulp is added to the fungal garden. Inmates of the colony also deposit their excreta on the growing fungal masses.

A fungal garden of parasol ants looks like a greenish grey felt with some rounded whitish spots. The spots are called kohlrabi. They are collections of terminal filaments of the fungus presenting bulbous swellings, which are named bromatia (Batra et al, 1967). Adults and larvae feed on the bromatia. The bromatia are sweet tasting, and rich in sugars, proteins and vitamins. Adults in the colony not only eat bromatia but also often just lick them.

One interesting fact about the fungus, associated with a parasol ant colony, is that it does not give out spore producing fruiting bodies or mushrooms. But the fungus in an abandoned nest is known in some cases to produce mushrooms. Perhaps some activity of the ants prevents the fungus from producing spores. One possibility is that the worker ants "prune" the fungal growth to prevent formation of fruiting bodies. It may also be that the ant excretion and saliva contain some substances, which prevent formation of spore producing parts, and induce development of bromatia. Whatever it be, this association of parasol ants and a fungus is a remarkable example of mutualistic or symbiotic living. While the fungus provides prepared food to the ants, the ants give to the fungus the advantage of wide dispersal without production of spores, and also a readily available substratum for its growth. A bacterium, which grows on the ant's bodies, produces antibiotics to kill a parasite that may infect their fungal crop (Schultz, 1999).

Cultivation of fungi for food by fungus-growing ants or Attini originated 50-65 million years ago (Mueller et al, 1998). The ants succeeded at domesticating multiple cultivars (553 cultivars have been isolated) and the mycelium cultivation has had a single evolutionary origin (Mueller et al, 2001). Relationships between the fungus and the ants are extremely specific and not yet properly understood.

kohlrabi bromatia bromatia

mycelia

— Fig. 35.2. Structure of bromatia under magnification.

— Fig. 35.3. A part of an orderly procession of parasol ants, moving on a tree limb, returning to the nest with pieces of leaves (after Batra and Batra, 1967).

— Fig. 35.4. A leaf-cutting ant of the genus Atta, carrying a bit of leaf. (after Linsemayer, 1973).

— Fig. 35.5. A dwarf worker (minime) cleaning an Atta soldier (after Weber, 1966).

References

Batra, S. W T. and Batra, L. R. 1967. The fungus gardens of insects. Scientific

American 217 (5): 112-120. Linsenmayer, W 1973. Insectes du Monde. Stock publs., Paris: 379 pp. Mueller, U. G., Rehner, S. A. and Schultz, T. R. 1998. The evolution of agriculture in ants. Science 281: 2034-2038. Mueller, U. G., Schultz, T. R., Currie, C. R., Adams, R. M. M. and Malloch, D. 2001. The origin of the attine ant-fungus mutualism. The Quarterly Review of Biology 76 (2): 169-197. Poole, L. and Poole, G. 1963. Weird and Wonderful Ants. Heinemann, London. Schultz, T. R. 1999. Ants, plants and antibiotics. Nature 398: 747-748. Weber, N. A. 1966. The fungus-growing ants. Science 153: 587-604.

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