282 PRINCIPLES OF ANIMAL BIOLOGY 



and so on. In those eases in which two or more species form galls on the 

 same species of plant, or in which galls are produced on plants used in 

 some other way by another species of animal, the gall-producing habit 

 leads to the formation of animal communities. 



Highly Specialized Relations of Animals to Plants. — A few instances 

 of the relations between animals and the plants that serve as their food 

 and perhaps protection are so intimate and mutually so advantageous 

 as to require special mention. These relations are quite as close, in the 

 examples to be described, as is the relation called symbiosis; but the term 

 symbiosis is ordinarily applied only to relations between animals or be- 

 tween plants. The interdependence in these cases is, indeed, so special- 

 ized that it does not lead to the formation of animal associations, for 

 in each instance onlj^ one species is thus related to a given plant. 



Moths and Yucca. Moths of the species Pronuba yuccasella live 

 upon the developing seeds of common Yuccas, and the success of the 

 moths depends on a plentiful supply of these seeds. Now, seeds in 

 plants develop only when the stigmas of the flowers are pollinated. 

 The moths appear deliberately to collect the pollen from the flowers and ap- 

 ply it copiously to the stigmas, insuring the production of the maximum 

 number of seeds. At the same time they lay eggs in the ovaries of the 

 plant where the seeds later develop. There are always more seeds than 

 are required as food for the moth larvae, so that subsequent generations 

 of the food plant are provided. Since the adult moth does not itself use 

 for food the pollen which it collects, the habit of gathering it and placing 

 it upon the stigmas has reference only to the success of the offspring. 

 Since one cannot ascribe forethought to the adult moths in this case, their 

 behavior is said to be instinctive; but the psychology of such remarkable 

 behavior has never been satisfactorily explained. 



Insects and Figs. A fig is a thick-walled hollow receptacle on the 

 inner surface of which numerous minute flowers are produced. The 

 fig does not attain its maximum size or best flavor unless these flowers are 

 pollinated so that seeds are produced, and pollination is wholly dependent 

 upon an insect, the fig-wasp Blastophaga. These insects enter the fig 

 at a minute opening in the end, for the purpose of laying their eggs, and 

 incidentally they carry pollen from some fig previously visited. The culti- 

 vated Smyrna fig is so constructed that the eggs of Blastophaga cannot 

 be laid in it, but the insects visit it in search of a suitable place, so that the 

 Smyrna fig receives the benefit of pollination without offering much in 

 return. The insects must come from inferior wild figs in which their 

 eggs can be laid, and it is the practice of fig-growers to hang wild figs 

 containing Blastophaga ready to emerge in the Smyrna fig-trees when the 

 flowers of the latter are ready for pollination. 



Relations of Animals to the Physical Environment. — That various 

 conditions in the environment participate in bringing about animal 

 associations is obvious. It may be said in general that many physical 



