248 SOME ASPECTS OF ECOLOGY 



shown that cellulose-digesting bacteria are constantly present in 

 relation with the intestine. Although it is held that the micro- 

 organisms referred to are symbionts, the part they actually play 

 in the physiology of nutrition is obscure, and knowledge of this 

 aspect of the problem is greatly needed. 



Mansour and Mansour-Bek (1934) in their review of this subject 

 conclude that the intra-cellular micro-organisms of xylophagous 

 insects cannot be considered as playing an important part in the 

 digestion of wood. The work of Ripper (1930), and of others, 

 appears to indicate that many more insects than was previously 

 believed possess the ability to digest various components of wood 

 by means of the action of their own enzymes, particularly 

 cellulases. Extra-cellular micro-organisms found in the intestines 

 of various insects likewise play no part, they believe, in breaking 

 down cellulose for the use of the hosts. These organisms appear 

 to be used by the latter as a source of food. It is evident that the 

 subject is not one which it is possible to dogmatise over in the 

 present state of knowledge, and micro-organisms have been too 

 readily credited with performing functions of which there is 

 still a lack of conclusive proof of their actuality. 



In many plant-sucking Hemiptera micro-organisms are likewise 

 constantly present, and they often bear close resemblance to yeasts 

 both in form and method of reproduction. As a rule they are 

 located in special " symbiont " cells termed mycetocytes, which 

 form a compact and very characteristic organ known as the 

 mycetome. The latter is lodged in the body cavity (usually in the 

 abdomen), and is unconnected with the digestive system by ducts 

 or other means. The physiological significance of these organisms 

 remains unsolved, and this subject, along with their origin and 

 incidence in different insects, is discussed by Buchner (1921) and 

 by Uichanco (1924). Infection of the host takes place through 

 the egg, and for the details of the process the recent papers by 

 Uichanco and by Schrader (1923) may be consulted. It is 

 generally believed that the micro-organisms profit by the 

 protection and nutritive material which they receive from their 

 hosts, and that they benefit the latter as absorbers of waste 

 products and excess food materials. Efforts have been made to 



I 



