BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



233 



influence on the animal components of the 

 community involved, according to popula- 

 tion numbers of the dominant plants, e.g.. 

 according to purity of stand. It is the rela- 

 tively pure stands of agricultural crop plants 

 that introduce the ready transmission and 

 increase of plant disease and of plant-eating 



Fig. 62. The pitcher-leaf, Nepenthes sp., 

 represents the most elaborate of the pitcher 

 plants. Model of a single "pitcher," with side 

 cutaway to show interior. (Courtesy of Chi- 

 cago Natural History Museum. ) 



insects. The less uniform but nevertheless 

 extensive stands of such dominants as cat- 

 tail or white pine in nature contrast radi- 

 cally with the complexity of the community 

 in which a mesophytic hardwood forest 

 forms the matrix. 



The bromeliad whorls, pitcher plants, 

 and tree holes afford striking examples of 

 minor niches. Others that immediately sug- 



gest themselves are the ripening individual 

 mushroom, the insect-bored leaf or plant 

 stem, the ant-attracting hollow stems and 

 thorns of plants, and the rotting log with 

 its invisible bacterial and mycelial living 

 components. These niches lead us directly 

 to the nest structures of animals, which on 

 one hand constitute a biotic modification of 

 the physical conditions of the environment, 

 and on the other tend to reduce biotic pres- 

 sure on the nest-building animal and, more 

 especially, upon its young. Nests may be 

 classified naturally into individual, family, 

 and communal types. Nest construction as 

 a response to the abiotic and biotic environ 

 ment is clearly an evolutionary phenome- 

 non (see pp. 425 and 633). Nests of all 

 kinds, from simple to elaborate, in addition 

 to their primary inhabitants, tend to acquire 

 a more or less specific assemblage of ani- 

 mals, such as those of a meadow mouse 

 nest, a prairie dog buiTow, or, in its most 

 elaborate development, a termite or ant 

 nest. An extreme of the biotic environment 

 as such is to be seen in the nest of the army 

 ant, composed of the hving workers (p 

 431). 



Phragmosis 



An extreme type of niche adaptation is 

 seen in the hole-closing devices of a great 

 variety of animals, whose principle of 

 operation was termed by Wheeler (1927. 

 p. 30) phragmosis. It is exhibited most 

 notably in certain ants and termites, in 

 which the head of a soldier is modified to 

 fit the openings in woody plants employed 

 by the insects. The device involves a series 

 of adjustments of behavior as well as of 

 structure. Wheeler writes: 



"These ants use the head, like the thick 

 door of a safe, to close the entrance of the 

 nest and keep out intruders. The nest which 

 is excavated in hard wood, ligneous galls or the 

 stems of rushes, has a perfectly circular en- 

 trance which is guarded by a soldier whose 

 head exactly fits the orifice. When a worker 

 desires to forage she strokes the soldier's 

 abdomen with her antennae and the animated 

 door moves back and as soon as she has passed 

 out of the nest returns at once to its previous 

 position. On returning slie knocks with her 

 antennae on the exposed truncated surface of 

 the janitor's head and a similar response per- 

 mits her to enter. I find this type of head in 

 single exotic species of three other unrelated 

 genera: Pheidole, Crematogaster, and Epopos- 

 truma, which, in all probability have much the 

 same habits." 



