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 sub- 

 tropical forests of the New World a peculiar tribe of ants 

 (Attini) have acquired the habit of making mushroom gar- 

 dens 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. 3 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 culti- 

 vated 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 grow- 

 ing 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. 



[219] 



