246 SOME ASPECTS OF ECOLOGY 



this category. Further species of micro-organisms are beheved 

 to be definitely symbiotic in that they play an important part in 

 the digestion of the food of their hosts and, at the same time, meet 

 their own nutritive requirements in the process. Micro-organisms 

 of this type are more especially present in insects feeding upon 

 dead or decaying animal and vegetable material, or in plant- 

 sucking insects. A large number of cases of reputed symbiosis of 

 this kind have been brought to light in recent years, but the 

 relationship has more often been accredited from inference than 

 as the result of definite physiological proof. For example, the 

 intracellular and intestinal yeast-like, conidia-like and bacteroidal 

 bodies present in so many insects, and transmitted from generation 

 to generation through the eggs, are usually regarded as symbionts, 

 but until such micro-organisnis can be studied apart from the 

 hosts, definite proof of their physiological role would seem to be 

 unobtainable. 



One of the best established cases of symbiosis between micro- 

 organisms and an insect host is afforded by the intestinal Protozoa 

 harboured by the lower termites. The present writer suggested 

 in 1919 that many of the peculiar forms of flagellate Protozoa 

 harboured by wood-feeding termites were not parasitic as 

 commonly stated. On the other hand, it appeared that they 

 derived their nutriment by ingestion of the wood-particles render- 

 ing the latter, in the process, assimilable as food by their hosts. 

 The important researches of Cleveland (1924, 1925) have proved 

 the truth of this contention and are of exceptional interest, since 

 they demonstrate that the principal constituent of the wood 

 utilised by the Protozoa is cellulose. He showed that the Protozoa 

 can be readily killed off, without injury to the termites, by 

 incubation at 36° C. for twenty-four hours, by starvation or by 

 increasing the oxygen pressure of the atmosphere. Termites thus 

 defaunated die within ten to twenty days if fed upon normal wood 

 diet, but when reinfested with Protozoa their ability to utilise 

 wood is regained, and they are able to live indefinitely. Cleveland 

 further demonstrated that termites are able to live and actively 

 multiply on a diet of nitrogen-free cellulose so long as Protozoa 

 are present. He, therefore, suggested that these insects must in 



