Termites are insects that collect vegetation, chew it up, and leave the chemical breakdown to other organisms. There are two strategies. The most primitive termites swallow the chewed vegetation and pass it to a fermentation chamber in their bodies. There, anaerobic bacteria break down the cellulose, an organic compound that forms about 33 percent of all plant matter. The termites are nourished by the ever-growing population of microorganisms in their guts that turn the grass, leaves, and twigs the insects ingest into glucose. Cattle do much the same thing: they allow bacteria to ferment the cellulose in an airtight rumen (digestive chamber), and then digest the bacteria. Termite evolution has several obvious trends, from primitive species, which live in small hidden colonies, to groups millions strong, the builders of enormous mounds that allow for heat and gas exchange. The less advanced groups digest microorganisms, which do the real work of breaking food down. The culture (colony) of cellulose digesters is passed along through a special exchange. Young termites feed on a special liquid secretion provided by adults, rich in the group's digestive heritage. When reproductive termites – those destined to produce offspring – leave the nest, they carry in their stomachs the microorganisms essential for the digestive success of their offspring. Treat a colony of these termites with an antibiotic solution, and they will slowly starve to death. More advanced species have a different feeding strategy. The energy source is still cellulose, but it is digested outside the termite's body. Not having to carry around large chambers of slowly fermenting cellulose solution makes these species more nimble and efficient. Foragers bring twigs and leaves back to special areas and chew them. They then transplant bits of fungus growing on other pieces of nearby vegetation onto the gnawed edges, where the fungi break down the cellulose. Fungi is the only kingdom of organisms able to digest cellulose in air, though they need warmth and humidity to do the job efficiently. This is just what the termites provide. Moreover, these social insects carefully tend the fungus-covered vegetation by treating it with antibiotics they secrete to keep bacterial growth to a minimum. When it is time for the fungus to reproduce, pieces are carried into the open to complete the life cycle. Some species of fungi are found only in termite mounds of a particular species; without their caretakers, these fungi would die. Needless to say, the termites eat the fungi; neither can live without the other. Reproductive termites even carry a chunk of fungi when they leave on mating flights. The evolutionary trend in termites is to forsake excavated nests in soil or wood, like those of most ants, for carton nests constructed inside excavations or on trees. (When referring to termites, carton means, broadly, nesting material consisting of a mix of adhesive saliva or feces with earth or pulp, and even sand, to create cells, floors, walls, graceful arches, tiered roofs, chimney stacks, and buttressed towers up to twenty feet high.) Primitive termites do not store food; they live from hand to mouth, inside a rotting tree, for instance. Advanced termites have special carton areas for food they hold in reserve; these supplies consist of nonperishable material such as grass clippings, analogous to the hay and straw fed to cattle in the winter, and are kept in a dry carton loft. Primitive species need wet cellulose, such as damp wood; more advanced species can also process dry material. To expand their niche in this way, dry-diet termites require a source of water. In arid habitats, they excavate vertical tunnels down to the water table, as much as 150 feet below, which fan out at the base to increase the area of contact and thus maximize the rate of subsurface water accumulation. Finally, less advanced termites remain their entire lives in tunnels and cells excavated in or near wood. More complex species, on the other hand, search for food away from a central nest. To ensure that they can work in safety, they burrow shallowly through the earth or build mud-covered tunnels on the surface of the ground or trees and around the food they wish to harvest.