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168 Foraging may be done via underground tunnels or thin-walled surface tubes, or in the open at night or on humid, overcast days. Many species release trail-marking pheromones (see Chapter 13, Section 4.5).

Cellulose is the primary component used by the termites whose midgut produces cel-lulase. In addition, to facilitate breakdown and use of the food, complex relationships have evolved between termites and microorganisms (protozoa, bacteria, and fungi). In all families except Termitidae, protozoa in the paunch produce a range of enzymes (including cellulase) that degrade the food into organic acids such as acetate and butyrate. In Termitidae anaerobic bacteria replace protozoa in the paunch, though the bacteria do not themselves break down cellulose. Within the Termitidae, members of the subfamily Macrotermitinae also culture a basidiomycete fungus of the genus Termitomyces in special "fungus gardens." Although the occurrence of these structures has been known since 1779, it is only quite recently that the precise relationship between the termite and fungus has been established. In a typical fungus garden the fungus grows on sheets of reddish-brown "comb" (decaying vegetable material) and is visible as a whitish mycelium containing conidia and conidiophores. This latter observation led early authors to suggest that the young termites were fed on the fungus, though it soon became apparent that the small amount of fungus would not satisfy even their requirements. It was some time before it was realized that the comb was a dynamic structure, being removed from below and built up on its upper surface or in the space beneath. In other words, the comb forms the food of the termites. Using staining techniques, it has been shown that the primary role of the fungus is digestion of the lignin component of the comb, releasing material that is then broken down by bacteria in the termites' gut. Secondarily, however, the fungus also provides vitamins and a source of nitrogen. Another point of contention was the method of comb construction. It was believed originally that the termites regurgitated chewed-up food to produce comb, but more recent work has shown that the comb is derived from feces. Thus, the vegetable material passes twice through the gut of Termitidae, a situation that is comparable with that in other termite families in which proctodeal feeding is an important method of extracting the maximum nutrition from the food (see below).

Only workers are able to feed themselves. Members of other castes and very young stages must be fed. Furthermore, their diet, as in other social insects, is different to a greater or lesser degree from that of workers. Exchange of food material (trophallaxis) occurs either by anus-to-mouth transfer (proctodeal feeding) or by mouth-to-mouth transfer (stomodeal feeding). The former method takes place in all families except the Termitidae, and normally it occurs only between workers or larger juveniles, although occasionally soldiers may act as donors. Proctodeal food is a liquid containing protozoans, products of digestion, and undigested food. Stomodeal food is either a semisolid material comprising the regurgitated contents of the crop, which are fed to soldiers in the lower termite families, or saliva, which appears to be the only food received by reproductives of all families, very young stages of lower termites, and all juvenile stages and soldiers of Termitidae.

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