304 ANIMALS AND THEIR ENVIRONMENT 



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Male 



Fetnstle 



Eirxgor^d female 



Figure 39.5. The common tick, Dermacentor andersoni. Eggs laid on the ground 

 hatch into six-legged larvae that feed on small mammals. These drop off, molt 

 into eight-legged nymphs that return to small mammals. ,'\fter molting on the ground 

 again the adults attack large mammals. The females become enormous after mating 

 and eventually fall to the ground to lay a thousand or more eggs. (After Chandler.) 



Colorado tick fever, Q fever, tularemia, undulant fever and several 

 forms of virus encephalitis. 



Bloodsuckers may serve as alternate hosts for the pathogens they 

 carry. The role of the mosquito in malaria has already been described. 

 Dog tapeworms use the dog flea as an intermediate host, and a few of 

 the nematodes pass parts of their life cycles in blackflies and horse- 

 flies. African sleeping sickness, a disease caused by protozoan parasites, 

 includes the tsetse fly as an alternate host, and leishmaniasis, a related 

 disease, involves sandflies. 



Paras/fes feeding on Living Tissues. Ectoparasites that feed di- 

 rectly on living flesh include trematodes, crustaceans, mites and fly 

 maggots. Many of these feed on blood as well as flesh. Certain trema- 

 todes parasitize the gills of fishes, crustaceans parasitize a variety of 

 animals including other crustaceans, annelids, molluscs, echinoderms 

 and fishes, and the mites and flies parasitize terrestrial vertebrates. Man 

 may be infested with the mange or itch mites (Fig. 39.6) that burrow 

 in the skin, or with chiggers, a mite that secretes enzymes which dis- 

 solve small holes in the host's skin for feeding. 



The maggots of several kinds of flies burrow in the skin of mam- 



