Ecology and Evolution - 577 



Fig. 30-5. The leaf butterfly, an outstanding example of protective mimicry. 

 (Courtesy of the American Museum of Natural History, New York.) 



between the parasite and its host. Seldom 

 can a particular parasite infest a wide variety 

 of different hosts, and frequently the para- 

 sitic relation is altogether species specific. 



Undoubtedly the adaptation of a parasite 

 to life on or in a host represents a gradual 

 evolution. By slow degrees the parasite de- 

 velops an increasing fitness to live on or in 

 the body of a particular host; and simul- 

 taneously the host develops countermeasures 

 to limit the growth of the parasite and to 

 neutralize its damaging effects. If the total 

 population of the host should succumb to 

 the ravages of a parasite, the parasite itself 

 would face extinction. Obviously, therefore, 

 the survival of a parasitic species depends in 

 certain measure upon the survival of the 

 host species. 



Effects of Parasitism on the Parasite. 

 Some parasites, especially ectoparasites (p. 

 178) like the mosquito, do not remain in con- 

 tinuous contact with the host species, and 

 such organisms do not display very drastic 

 adaptations to the parasitic habit. The mos- 

 quito, for example, is an insect that has 

 mouth parts that are plainly designed for 

 piercing the skin and sucking blood from the 

 host; and the saliva of the mosquito con- 

 tains an anticoagulant, which prevents the 

 blood from clotting as it is being imbibed. 

 But otherwise the mosquito displays little 



divergence from other closely related insects. 

 Other ectoparasites, however, maintain a 

 closer contact with their hosts, and these dis- 

 play more drastic adaptations. The flattened 

 body and spiny legs of the flea, for example, 

 enable this parasite to force its way through 

 the matted hair of the dog; and the clasping 

 talons of the louse (Fig. 30-6) make it diffi- 

 cult for this pest to be dislodged from the 

 surface of the skin. In extreme cases, indeed, 

 the fixation of a parasite to the host may be 

 so complete that the parasite loses virtually 

 all capacity for independent life. For exam- 

 ple, Sacculina, a parasitic crustacean, dis- 

 plays so drastic a degeneration of its eyes and 

 other sensory structures, and such an atrophy 

 of its locomotor organs, that it bears very lit- 

 tle resemblance to closely related species. 

 However, Sacculina displays free-living larval 

 stages by which it can be identified as a true 

 crustacean. 



Obligatory parasites may encounter seri- 

 ous difficulty at times when it is necessary to 

 abandon one host and find access to another. 

 In fact there are various adaptations that 

 facilitate the transfer of parasitic species from 

 host to host. However, only two such adapta- 

 tions will be considered, namely: (1) a tend- 

 ency of parasitic species to produce vast num- 

 bers of offspring, and (2) the ability of many 

 parasites to invade one or more intermediary 



