248 



ANALYSIS OF THE ENVIRONMENT 



fungi found in various heteropterous bugs 

 and in all the Homoptera. The inference 

 that these are mutualistic rests on the 

 development by the insects in question of 

 elaborate structural and physiological mech- 

 anisms for the transmission of the fungi or 

 bacteria into the maturing egg, thus ensur- 

 ing the transmission of the symbiont plant 

 from generation to generation of bug. 



It is to be further noted that this type of 

 mutuahsm is associated with the limited diet 

 of plant sap that characterizes the feeding 

 of the Homoptera. 



The presence of bacteria-containing struc- 

 tures in the bedbug and on the keratin-eat- 

 ing Mallophaga, again together with struc- 



the elaborate and varied structures for the 

 transmission of the bacteria or fungi devel- 

 oped in the host animal seem to exclude 

 the parasitic relation, in which the problem 

 of transmission falls to the parasite and in 

 general depends on chance or is met by 

 multiple host parasitism. For a review of 

 plant-animal mutualism, both external and 

 internal, reference should be made to the 

 comprehensive summaries of the subject by 

 Buchner (1930) and Steinhaus (1946). 



A further major type of plant-animal mu- 

 tualism is represented by the adaptations of 

 flowering plants to attract insect and other 

 animal visitors, and the complementary 

 structures and habits of animals that ensure 



Fig. 66. Transmission apparatus for symbiotic bacteria in the trypetid fly, Dacus oleae, 

 shown in sagittal section, A, of the ovipositor; R, rectum; O, oviduct; and C, caecae. The 

 caecae are packed with bacteria derived from the gut; the eggs are smeared with bacteria as 

 they pass through a longitudinal slit connecting the rectum and oviduct. Infection of the 

 individual egg takes place through the micropyle. B, Cross section through caecal region to 

 show slit connecting the rectum and oviduct. (After Buchner.) 



tures for obtaining the transmission of the 

 bacteria via the egg, aflFord further examples 

 of this somewhat obscure symbiotic rela- 

 tion. Numerous Diptera and Coleoptera 

 likewise exhibit bacterial and saccharomy- 

 cete-filled organs and have associated struc- 

 tures for transmission of the plant via the 

 egg to the succeeding generation. The sub- 

 ject of internal plant-animal symbiosis, and 

 especially the conclusive demonstration of 

 its mutualistic nature, ofiFers intriguing prob- 

 lems for research (see p. 712). One of the 

 more provocative of these problems, par- 

 ticularly in the present state of knowledge 

 concerning insects and vitamins, springs 

 from the evidence that these contained or- 

 ganisms supply essential vitamins of the B 

 complex for certain insect hosts (Blewett 

 ^nd Fraenkel, 1944). It is to be noted that 



cross pollination. Observation of the fact of 

 cross fertilization of plants by insects must 

 long antedate scientific studies of the phe- 

 nomenon. The whole subject was summar- 

 ized by Knuth in 1898, after classical stud- 

 ies by Darwin and by Fritz and Herman 

 Miiller. The present account leans primarily 

 on Doflein. 



The numerous structural arrangements 

 that make difiicult or prevent self-fertiliza- 

 tion in the monoclinous flowers of the 

 higher plants are evidence of a powerful 

 evolutionary advantage favoring cross ferti- 

 lization, and from the great variety of adap- 

 tive strjctures of insects it seems equally 

 evident that the food surpluses (p. 236) 

 provided by the excess pollen of plants, 

 with the addition of nectar, have been a 

 dominant factor in insect evolution (p. 



