114 COACTION: THE INTERRELATIONS OF ORGANISMS 



porary nests. Examples of this among lepidopterous larvae and spiders 

 are common. (See Figs. 25, 26, and 27.) 



The trunks of trees when hollow afford home sites for small mam- 

 mals, reptiles, and insects. Comb-making bees often utilize such cavi- 

 ties. A goodly number of Coleoptera and a few Lepidoptera and Hy- 

 menoptera burrow into dead wood, which serves as shelter and for 

 most of the insects it also supplies food. The larvae of click beetles 

 and the larvae and adults of rove beetles, ground beetles, etc., which 

 also are predatory, find shelter in dead wood. A few Hymenoptera 

 make tunnels into wood in which to lay their eggs, but they are not 

 important as regards influence. 



REPRODUCTIVE AND SOCIAL COACTIONS 



The reproductive coaction involves mating or the fertilization of 

 the eggs, which calls forth aggregations of individuals. The swarming 

 of marine worms is one of the outstanding types of assemblage for the 



Fig. 28. — Defense circle of the musk ox {Ovibos moschatus Zim.)- It is an excel- 

 lent defense against wolves and also affords protection to the young. (Sketched 

 from a photograph by D. B. MacMillan in a report by American Committee 

 for International Wild Life Protection, 1934.) 



lower invertebrates; this brings individuals into close contact and 

 permits fertilization of eggs cast into the water. In terrestrial commu- 

 nities the choosing of mates, sometimes accompanied by fierce contests 

 between males, is a well-known coaction. 



The care of young is often of much greater importance in the com- 

 munity than nest building in itself, as it involves a greater drain on 

 the food supply. This is especially true among birds, which collect 

 enormous amounts of animal food to nourish their rapidly developing 

 young. The hiding of the young by many mammals is important and 

 varies as to method. Certain rabbits secrete the young individually, 

 the litter being scattered about; this is usually a shelter coaction. The 



