INSECT ECOLOGY 379 



always abundant, like the pawpaw, to which the Ajax butterfly is 

 confined; or it may not be. 



In a state of nature, if food plants of one species are scattered among 

 other plants, their insect enemies do not become injurious; but in a 

 state of nature, if many plants of one species grow together, insects may 

 injure them (in forests, for example). Where man grows one kind of 

 plant over a large area, insect enemies flourish (Hessian fly, chinch bug, 

 cotton boll weevil, codling moth, etc.). 



The same relations exist between parasites or predators and their 

 hosts. A parasitic species of insect that is limited to one species of host 

 will die if it destroys all the individuals of the host species. The 

 successful parasites (as Ichneumonidae, Chalcididae, and Braconidae) 

 are those that have available immense numbers of a single species of 

 host, or a large number of species as hosts. 



Most predaceous insects, however, feed indifferently on almost any 

 species of insects that they can overcome, and often do not limit them- 

 selves to insects for food; therefore their numbers are not affected by the 

 absence of this or that species of possible victim. 



These food relations form a most important factor in the interactions 

 of organisms, the subject next to be considered. 



4. BIOTIC CONDITIONS 



The animals and plants of a region form a vast complex, in which 

 every organism affects every other, directly or indirectly, and is in turn 

 .affected by all the others. Furthermore, all the organisms are influenced 

 by their environment, and in turn affect the character of the environ- 

 ment itself more or less. All the organisms are bound up with one 

 another in an intricate network of interactions which the mind can only 

 partially comprehend. 



Interactions. As a familiar illustration of these interactions, take 

 the case of any common plant louse and the extensive society, or conso- 

 cies, which it dominates. 



To begin with, the numbers of aphids depend greatly upon inorganic 

 influences, as heat or cold, dryness or moisture; evaporation being 

 important. Aphids are often blown off their plants, or washed off by 

 rains, and killed mechanically. When they are abundant, many are 

 squeezed to death between branches that are blown against each other. 

 Bacteria and fungi destroy the lice. The fungus, Empusa aphidis, is 

 the most important enemy, for in damp weather it can almost extermi- 



