BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



247 



enormous subterranean nests by the workers 

 streaming back from some tree or other 

 plant that is being stripped of its leaves. 

 The leaf fragments decompose in heaped- 

 up masses in special chambers in the nest 

 to form mushroom beds. The transmission 

 of the fungus to a new colony is accom- 

 plished by the virgin queen, who carries 

 with her a pellet of fungus from the old 

 nest in a special pocket situated below the 

 mouth, and after mating deposits it in the 

 new small chamber in which her first eggs 

 are laid. 



The fungus-growing termites establish 

 special chambers in their large terrestrial 



great variety of other animals is particularly 

 significant in forms with a restricted diet, 

 and especially in those restricted to a diet 

 of cellulose. True mutualism is inferred in 

 many such relations, and is experimentally 

 demonstrated as mutual interdependence in 

 others (see p. 716). Especial attention has 

 focussed on the biological aspects of the 

 protozoan-cockroach and the protozoan re- 

 lationship with the less highly evolved ter- 

 mites (Fig. 254), in which the essential 

 function of cellulose digestion by the re- 

 markably distinctive protozoans is most 

 clearly developed. A similar, but less ac- 

 curately definable, mutualistic function and 



Fig. 65. Intestinal caecae of Hemiptera, showing extreme elaboration of the structures con- 

 taining the supposedly mutualistic bacteria. A, Anasa tristis; B, Thyrecoris unicolor; and C, 

 Blissus leucopterus. In all figures c designates caecae; i is the ileum; M', M', M', and M' refer 

 to the first to fourth stomachs; MT refers to the malpighian tubes; and R is the rectum. (After 

 Galloway. ) 



nests and grow fungus on a substrate of ter- 

 mite excrement. The mode of estabhshment 

 of the fungus garden in a new colony is 

 unknown. 



In these several types of fungus-cultivat- 

 ing and fungus-eating insects the insect may 

 be said to live in a fungus environment. In 

 these instances the fungus relations diflFer 

 sharply from those of the varied inhabitants 

 of the fungus niche, in which fungi provide 

 both food and shelter for a whole series of 

 insects, nematodes, and other animals. The 

 step toward growing and controlling the 

 growth of a fungus as an invariable food 

 supply falls into line with other tendencies 

 toward control of the environment that are 

 most notably associated with the develop- 

 ment of societies. 



The symbiosis of gut-inhabiting bacteria 

 and protozoa with vertebrates and with a 



relation to cellulose digestion seems to be 

 the role of a great many bacteria and some 

 protozoa that constitute a part of the flora 

 and fauna of herbivorous animals— especially 

 in the rumen of artiodactyls and the caecum 

 of lagomorphs and rodents. Bacteria inhabit- 

 ing these organs and other parts of the in- 

 testines produce significant quantities of 

 various B vitamins that are utilized by their 

 hosts. Man is one of the many animals 

 showing such relationships with his intes- 

 tinal flora (Najjar and Barrett, 1945). 



The similar phenomena in insects result 

 in elaborate modifications of the gut to pro- 

 vide special structures in which bacteria 

 may be lodged (Fig. 65). 



The evolutionary step is not great from 

 the last-mentioned type of organized asso- 

 ciation to the truly internal nodules and 

 special structures containing bacteria and 



