THE EVOLUTION OF ANTS
with this advance in diversity of form, there is a notable change in feeding habits, the primitive forms being purely carnivorous, like their ancient wasp-like ancestors, and the more advanced types having become increasingly vegetarian. The vegetable feeders are best developed in regions where competition for insect food is keenest—that is, in deserts, where insect food is scarce or limited to a short season, and in the tropical rain forests, where the ants must enter into close competition with many other predatory insects and with insectivorous reptiles, birds, and mammals. In the deserts of the world (in southwestern United States, Mexico, Sahara, South Africa, Central Australia) we find that two kinds of ants have become adapted to a vegetarian diet, the harvesting ants, which feed largely or exclusively on the seeds of plants, and the honey-ants, which store in the crops of a special caste of worker a sweet liquid ("honeydew") collected from plant-lice, scale-insects, and oak-galls. In the tropical and subtropical forests of the New World a peculiar tribe of ants (Attini) have acquired the habit of making mushroom gardens in which they grow fungi as food. The garden beds are made of pieces of leaves, which they cut from the trees, or from the collected excrement of caterpillars or other insects that feed on plant tissues (Figs. 5 and 6). Fully a hundred species of these attine ants are known, and some of the larger species are at times very injurious to the agriculturist and the horticulturist, because they use the leaves of cultivated plants (sugar-cane, orange trees, etc.) as material on which to grow their food-fungus. Many of the most highly specialized termites, or "white-ants," in the Old World tropics have independently developed a similar habit of growing fungi. Among these insects, however, the substratum of the fungus gardens consists of triturated wood, which has been passed through the intestines of the workers.
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