ORGANIC RESPONSE 



281 



that it is very difficult to discern the Caprella on the 

 hydroid colony even when one is looking for them. 

 To the safety afforded by this protective resemblance 

 may be added the protection of the stinging nettle 

 cells of the hydroid. Such a relation, which profits 

 but one party 

 to the associa- 

 tion and on the 

 other hand does 

 not injure the 

 other, is termed 

 mutualism or 

 commensalism. 

 In other, cases 

 both sides profit 

 by the connec- 

 tion. A very 

 familiar exam- 

 ple is the rela- 

 tion that exists 

 between the 

 aphids and 

 ants. The 

 former secrete 



11 1-1 u FIG- 100- Rose Aphids visited by ants, natural 



a jelly-like sub- size from u _ (From Kellogg .) 



stance called 



" honey-dew," which the ants are very fond of, and 

 in order to maintain a supply, the ants keep the 

 aphids in flocks, as men do cattle, tending them, 

 carrying them back if they stray, and " milking " 

 them by stroking them with their antennae, thereby 



