98 ECOLOGY Part II 



in being so seriously dependent upon the special diet of warm blood that the 

 species will die without it. Male mosquitoes do not show any such trend to- 

 ward the habit; they still drink fruit juices. Fleas and sucking lice represent 

 steps in increasing parasitism in the persistence with which they stay on their 

 host. Fleas stay on a dog most of the time; they also frequently jump off. Lice 

 stay on except by accident. Their claws lock onto the hairs of the mouse or 

 other host and they cling fast as fleas never do. Chiggers go still further. They 

 are the parasitic larvae of certain kinds of mites that actually burrow into the 

 skin (Fig. 6.6). 



The parasites so far mentioned are a few of the great host of ectoparasites 

 that attack the outsides of animals and represent the earlier stages of para- 

 sitism. Endoparasites spend most of their lives inside the bodies of animals 

 and represent the extremes of adjustments to parasitic living (Fig. 6.7). The 

 easiest way for an endoparasite to enter an animal is by way of the mouth 

 along with food or drink. Other possible entrances are into the breathing 

 organs, the excretory ones, the reproductive organs, and through the skin. 



Life Histories. Whatever their habit, animals go through various phases 

 during their life spans. The embryo of any animal is very different from the 

 adult; young animals may live in one environment and later move to a very 

 different one. Parasites often change from one host to another while in their 

 egg or larval phase of life. This is especially difficult for endoparasites which 

 have to take advantage of the habits as well as the structure of their second 

 hosts in order to enter them. 



A parasitic animal may pass directly from one host to another of the same 



Fig. 6.7. Endoparasites; phases in the life of two endoparasites in which parasit- 

 ism is highly developed. Left, trichina worms: Trichinella spirella, coiled and 

 dormant among muscle cells, an example of the phase of waiting, characteristic 

 of many endoparasites. Right, trypanosomes: Trypanosoma gambiense, a proto- 

 zoan blood parasite. (Fig. 21.10, trypanosomes and West African sleeping sick- 

 ness.) They reproduce in enormous numbers in the blood of man and in the big 

 game of Africa and are transmitted by the tsetse fly. The multiplicity of their 

 populations and dependence upon a second transmitting host are characteristic 

 of many endoparasites. (Courtesy, General Biological Supply House, Inc., Chi- 

 cago, 111.) 



