PAKASITISM 317 



in the intestines of animals are for the most part harm- 

 less mutuals that derive their nourishment from the 

 host without causing any harm. 



Certain bees live under conditions of mutualism as 

 Psithyrus in the nests of Bombus. It is supposed that 

 the only benefit Psithyrus receives is the shelter of the 

 nest, but it may be that its young are nourished by the 

 bees' honey. In return for this, Psithyrus aids in 

 defending the nest. 



Helotism. In this form of symbiosis the one organism 

 is supposed to enslave the other and enforce it to labor 

 in its behalf. The term seems to be susceptible of many 

 applications in the hands of different writers. Thus, 

 by some it is said that among the lichens the algae are 

 enslaved by the symbiotic fungi. A better example 

 is to be found in the behavior of certain ants that under- 

 take systematic campaigns against other ants bringing 

 home the conquered to be their slaves. The enslaved 

 ants soon seem to be quite at home and maintain a 

 subsequent harmonious symbiotic existence in the nests 

 of their masters where they are easily recognized by 

 their different specific characters. So far as known, it is 

 only the worker ants, never the males or females, that 

 are thus enslaved, and having no sexual instinct, but 

 only the instinct to labor, after the heat of the action is 

 over, they seem to be as contented to work in one place 

 as in another. The precaution seems to be taken, how- 

 ever, to have them participate in the household duties 

 rather than to engage in foraging expeditions or to join 

 the ranks in warfare. 



Parasitism. This is a form of symbiosis in which one 

 symbiont receives advantage to the detriment of the 

 other. The symbiont receiving the advantage is known 

 as the parasite, the other as the host. 



The parasitic relationship is based upon the ease with 

 which the products of another's labor can be seized upon 

 to the saving of one's own expenditure. In most cases, 

 therefore, the parasite is the lazy creature that lives at 



