250 SOME ASPECTS OF ECOLOGY 



in relation with their hosts. The most important work in recent 

 years is that of Aeschner and Ries (1933) and Aeschner (1932, 

 1934) with reference to the body louse, Pediculus. The symbionts 

 in this creature are localised in a small organ situated in the 

 mid-ventral line of the abdomen rather nearer the anal end of 

 the body. The complete life-cycle of these micro-organisms has 

 been worked out by Ries (1932). Aeschner (1932) and Aeschner 

 and Ries (1933) were able to show that extirpation of the mycetome 

 when filled with symbionts caused marked deficiency symptoms 

 in the louse. If, however, extirpation took place after the micro- 

 organisms had migrated to the oviduct it had no influence on the 

 well-being of the insect. From these results it was concluded 

 that it was not the operation, or the absence of the mycetome, 

 but rather the absence of the symbionts that induced the deficiency 

 symptoms. In his later work Aeschner (1934) studied the effect 

 of centrifugalisation on the eggs of the louse. It appears that 

 the symbionts are eliminated in the process and are wanting in 

 the young insects. The latter fed and developed normally until 

 the fifth or sixth day, when they died. This result was in complete 

 accord with that obtained by extirpating the mycetome, and 

 it is concluded that the relationship of micro-organisms and 

 host is one of true symbiosis. What role these organisms play 

 in Pediculus is unknown, but it is possible they are concerned 

 with the production of some " vitamin " or growth-promoting 

 substance. 



Blood-sucking insects regularly contain intracellular micro- 

 organisms, and Wigglesworth (1936) describes a bacterial organism 

 which occurs constantly in Rhodnius proliccus. Hobson found 

 that sterile blood was a deficient diet for blow-fly larvae unless 

 " vitamin B " was added. This same investigator (quoted by 

 Wigglesworth) has carried out a few experiments adding the 

 bacteria from Rhodnius to sterile horse serum and then feeding 

 Lucilia larvae on the mixture. The results appear to point to 

 the conclusion that when blood is so infected it becomes an 

 adequate diet for insect growth. These observations lend support 

 to the view that symbiotic organisms, in exclusively blood-sucking 

 insects, provide an endogenous source of vitamin. 



