THE SOCIAL INSECTS 



The termites which are more especially wood feeders have 

 a remarkable digestive specialisation. Their intestine con- 

 tains vast numbers of protozoa belonging to a special group 

 which is found only in this situation. These microscopic 

 unicellular animals are able to digest the cellulose which 

 forms the major part of all woody tissues. They convert the 

 cellulose into sugars or into other substances which the 

 termites can assimilate. Although wood might provide a 

 complete diet, it would be an extremely wasteful one if 

 none of the cellulose could be absorbed. The relation be- 

 tween termites and their intestinal fauna is a typical example 

 of a symbiosis : the protozoa receive chewed wood in a 

 suitable environment, while the termites receive back digested 

 cellulose and also assimilate a fair proportion of the protozoa 

 themselves. 



The young brood quickly become infected with the 

 protozoa by eating the faeces of older members of the colony, 

 and the royal couple carry some infected material with them 

 when they go on their marriage flight. The process has an 

 interesting analogy with the digestion of such animals as 

 the cow, in which the rumen, a special compartment of the 

 intestines, contains vast numbers of bacteria which help to 

 digest grass or hay. 



About twenty-five years ago, Cleveland showed that the 

 protozoa in the termite intestine could be eliminated by 

 exposing the insects to a raised pressure of oxygen. After 

 this has been done the termites can no longer assimilate 

 cellulose, and, if given no food but purified paper, die rather 

 rapidly. Conversely, normal untreated termites can subsist 

 on such a diet for a long time. Those termite species which 

 seem to have a more varied natural diet lack the intestinal 

 fauna ; exactly what fraction of their diet they assimilate and 

 how they do it is unknown. 



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