570 



INTERRELATIONSHIPS OF ORGANISMS 



Only slightly less intimate relationships exist between many species of 

 plants and of insects. The insect visitors to flowers are rewarded by nectar 

 or pollen that in many instances forms the insects' staple, or even sole, 

 food supply. In return, the insect, in its journey from blossom to blossom, 

 carries pollen from anthers to pistils and so effects the fertilization of 

 the plant's ova. So close is this relationship that a great many species 

 of both insects and plants are wholly dependent upon this mutual ex- 

 change for their continued existence. Many of the 

 specific structural characters of both insects and 

 flowers and many of the specific psychological 

 behaviors of insects are special adaptations to some 

 particular insect-flower interdependency. 



Various intermediate relationships also occur, in 

 which one member receives much more than 50 per 

 cent of the total benefit. Nearly every ant and termite 

 nest houses a number of so-called "guests" — other 

 species of arthropods that live in the nest, and that 

 vary in role from more or less innocuous scavengers 

 to definite social parasites living on the food gath- 

 ered by the ants for their own use, much as rats, 

 mice, and roaches live at the expense of man. Such 

 social parasitism, commensalism, may be combined 

 with other, and symbiotic, relationships, and it is 

 not always possible to distinguish between the two. 

 Another rather special type of interspecific relation- 

 ship is exemplified by the "slave-making" habits of 

 certain ants. These slave makers raid the nests of 

 certain other ant species, kill or drive off the adult 

 ants, and carry back to their own nests the young 

 (pupae) of their victims. These young, when they 

 mature, become a part of the social organization in 

 the nests of their captors and perform a number of 

 essential duties for the maintenance of the group. 

 Although cooperative relationships among individuals of the same 

 species reach their highest development among ants, termites and men, 

 representatives from nearly every order of the vertebrates and arthropods 

 show at least some degree of social behavior. In many instances such 

 intraspecific cooperation is seasonal, as in the hibernating assemblages 

 that are formed in various species of insects, amphibians, and reptiles, 

 the winter herds of deer that band together for protection, and the wolf 

 packs that gather for winter hunting. Other groups appear to be nearly 

 or wholly permanent, as the beaver colonies that build and maintain 

 dams, houses, and canals, and the prairie dog "towns" of hundreds or 



Fig. 33.4. The root 

 system of a young 

 bean plant, with nu- 

 merous tubercles or 

 nodules in which ni- 

 trogen-fixing bacteria 

 live symbiotically 

 with the green plant. 

 (From Haupt, An In- 

 troduction to Botany.) 



