Chap. 6 MUTUAL RELATIONSHIPS OF ANIMALS 95 



Fig. 6.4. A minute beetle, Termitonicus mahout, that rides on the heads of the 

 workers of the termite, Velocitermes beebei, and takes bits of the food as it is 

 passed from one worker to another. An example of symbiosis, a general term that 

 includes a variety of partnerships. (Redrawn after Allee et ah: Principles of Ani- 

 mal Ecology. Philadelphia, W. B. Sanders Co., 1949.) 



One of the most remarkable examples of mutualism is that between wood- 

 eating termites and certain species of protozoans. The protozoans live pro- 

 tected within the intestines of the termites and in turn actually digest their food 

 for them. Bits of the cellulose food are taken in by the protozoans and changed 

 to sugar (dextrose) which is squeezed back into the intestine and absorbed by 

 the tissues of the termite. Experiments have shown that termites cannot sur- 

 vive long without the protozoans unless they are given a diet other than cellu- 

 lose. On the basis of the evolutionary history of termites it is estimated that 

 these intestinal intimacies have existed for 150 million to 250 million years. 



Parasitism. Another form of symbiosis in which an organism lives on or in 

 and at the expense of a larger plant or animal, called the host, is parasitism. 

 The parasitic mistletoe grows on a tree, commonly an oak. Animal parasites 

 are always small in comparison with their host and usually numerous. The 

 parasite obtains food, protection, or transport from its host, often all three of 

 these. 



Parasitic animals are discussed in the chapters dealing with the groups to 

 which they belong. These are especially: Chapters 21, Protozoa (sporozoans, 

 e.g., malaria); 25, Flatworms (tapeworms, et al.); 26, Roundworms (trichi- 

 nae, hookworms, et al.); 28, Annelids (leeches, et al). 



The relationship of parasitism costs the host its substance and the parasite 

 its independence. People who must have special food are restricted in their 

 travels; so are fleas and bedbugs. 



The Host, a Living Habitat. Plants and animals have three major dwelling 

 places: terrestrial — on or in the earth's crust, aquatic — in fresh or salt water, 

 and on or in living organisms. 



Parasites occupy living habitats. In them there are special places in which 

 various parasites thrive, such as the skin or the liver, just as different seashore 

 animals thrive in tide pools or in mucky sand. 



Living habitats offer ready food and protection, within limits. Parasites must 



