gig ANIMALS AND THEIR ENVIRONMENT 



becomes more probable, such parasites can rapidly produce extremely 

 high infection rates. 



In many cases where the parasite is found in more than one kind 

 of host, reproduction takes place in all hosts. The mosquito that picks 

 up a few infective malarial parasites from one person shortly has 

 enough parasites to infect all of the people it may bite after that. Simi- 

 larly,\he trematode miracidium lucky enough to get from its vertebrate 

 host to a snail reproduces so as to produce many cercariae, not just 

 one. The eggs of many of the insect parasites of other insects go through 

 a process called polyembryony to produce a number of larvae from 

 each egg that successfully reaches a new host. 



The remarkable rate of reproduction, often at more than one point 

 in the life cycle, makes it difficult to control parasites. Although all but 

 a few parasites may be eliminated by intensive medical treatment, those 

 few can shortly replace the entire population. 



Selective Modificafion of Organs. The intimate association of a 

 parasite with its host may eliminate the usefulness of certain of its 

 organs. The selective disadvantage of some structures is obvious, such 

 as the cumbersome wings of ectoparasitic insects that crawl through 

 feathers or fur. Useless structures tend to become reduced or absent in 

 parasites because there is no longer any positive selection in their favor 

 and the gene complexes responsible for their existence gradually are 

 dispersed. Such is presumed to have been the fate of the digestive tracts 

 of tapeworms and acanthocephalans. A mouth, gut and digestive glands 

 are not required for the survival of an organism living in the host's 

 digestive tract, where saprozoic nutrition is possible. 



Locomotor organs may also be useless. Most adult tapeworms do 

 not move again once the head is attached, and these tapeworms have 

 such poor musculature that they cannot crawl effectively. Although 

 larval parasitic copepods and barnacles have typical larval legs in the 

 free-swimming stage, in many species the legs rapidly disintegrate as 

 soon as the individual attaches to its host. Even the protozoan Sporozoa 

 have lost their original locomotor organelles. Most of the ectoparasites, 

 however, have fair to good locomotor organs, and the insects that have 

 lost their wings still have well developed legs. 



Sense organs become somewhat less useful as the locomotor organs 

 of ectoparasites decrease in size. Fleas, which have strong jumping legs, 

 have well developed eyes and an excellent sense of the warmth of 

 mammals at a distance. The latter is shown by waving first a cold 

 object and then the hand past fleas on the floor. The fleas show little 

 response to the cold object, but as the hand approaches they all turn 

 to face it and then jump upon it at the appropriate moment. Lice have 

 weak legs and most of them are blind, but they retain a good chemical 

 sense for use as they crawl over the host. 



Internal parasites have even less use for eyes, ears and other sense 

 organs. The only sense found in many internal parasites is a little 

 understood ability to migrate to a specific portion of the body, which 

 is presumably a form of chemical sense. Internal parasites with complex 

 life cycles including a free-living stage, such as the miracidium and 



