258 



ANALYSIS OF THE ENVIRONMENT 



host. This is common but not universal 

 among protozoan parasites. Often the sex- 

 ually immature stage is spent in an inter- 

 mediate host, or there may be a succession 

 of intermediate hosts. In a much-cited in- 

 stance, the broad tapeworm of fish and man 

 {Diphyllobothrium latum) passes through 

 at least three hosts. The egg is shed into 

 fresh water and develops into a ciliated, 

 free-swimming coracidium larva. This gains 

 entrance to a copepod and develops into a 

 small procercoid larva. If the infected cope- 

 pod is eaten by one of several species of 

 fishes, the procercoid develops into an 

 actively migrating plerocercoid stage. This 

 may be found in fish-eating pike and pick- 

 erel and sometimes in other predaceous 

 fishes, in the northern United States. The 

 sexually mature, strobilating tapeworm is 

 found in man, or in several other fish-eating 

 mammals (Pearse, 1942; Craig and Faust, 

 1943). 



Another type of specificity is illustrated 

 by many parasites, especially by Ascaris, 

 and other worms, or the larvae of the var- 

 ious botflies. Restriction to special habitats 

 within the host is the rule rather than the 

 exception for these and many more. Thus 

 Ascaris, when adult, fives in or near the 

 duodenum. The eggs of the horse botfly 

 (Gasterophilus equinus) hatch in the stom- 

 ach, and the larvae attach to its wall, 

 while the eggs of a common botfly of cattle 

 hatch on the Hmbs; the larvae then pene- 

 trate the skin and wander through the body 

 tissues to come to rest along the back, on 

 each side of the midfine. This type of speci- 

 ficity holds both for ectoparasitic Mallo- 

 phaga (Kellogg, 1913) and for endopara- 

 sites, including the majority of parasitic bac- 

 teria (Smith, 1934). 



Host specificity usually refers to the tend- 

 ency of many parasites to attack a single 

 species or a Umited number of taxonomi- 

 cally related species. There appears to be 

 a widespread befief in host specificity of 

 this kind but it is diflBcult to find a definite 

 statement to that effect in the several gen- 

 eraHzed books on parasitology that were 

 examined. Chandler (1944) is cautious. 

 "Every parasite," he says, "has at least one 

 species of host, and sometimes several in 

 which it can meet fiving conditions." Wen- 

 rich (1935, pp. 606, 643') is frankly 

 skeptical. In his three decades of experience 

 in studying protozoan parasitism, he has 



* And personal communication in 1944. 



matured a conviction that the idea of host 

 specificity has too many exceptions to make 

 it a significant principle in parasitology. The 

 other side of this question will be developed 

 in another section (see p. 628). 



Wenrich's account of protozoan parasiies 

 indicates "(1) that in many instances the 

 same or nearly related species have invaded 

 many hosts belonging to widely different 

 taxonomic groups; (2) that a number of 

 species of the same genus may be found in 

 the same species of host and (3) that one 

 species of host may harbor many species of 

 parasites belonging to widely different 

 groups." 



Insect parasites show both specificity and 

 nonspecificity in their toleration of hosts. An 

 insect may show avoidance of apparently 

 potential hosts, the opposite of host selec- 

 tion (see p. 615). Such avoidance may be 

 to all organisms other than those of a single 

 taxonomic unit, which may be as restricted 

 as a species; more usually the avoidance is 

 related to some higher taxonomic category. 

 Thus the braconid subfamily Aphidiinae 

 shows such "host avoidance" except to 

 aphids. Near the other extreme, the tachinid 

 fly Compsilura concinnata has been re- 

 corded from more than 200 species of hy- 

 menopterous and lepidopterous larvae in 

 the United States and from more than fifty 

 European species (Wardle, 1929). 



An interesting footnote to the discussion 

 of host-parasite specificity is furnished by 

 the distinct nonspecificity of the relations 

 between biting bird fice (Mallophaga) and 

 the birds of the Galapagos Islands. The 

 general rule, Kellogg (1913) said, is that 

 the Mallophaga of one host group, such as 

 genus, family, or order, are more or less 

 closely confined to each particular group 

 and tend to be characteristic of it. This rule 

 breaks down for Galapagos birds, because, 

 Kellogg suggests, the land, shore, and sea 

 birds in that region meet in close contact 

 with each other on the shore sand and 

 rocks. The unusual opportunity for transfer 

 from one host to anoAer of widely different 

 taxonomic position and different ecological 

 habitat in other parts of the world helps to 

 account for this particular lack of host spe- 

 cificity. 



Other instances of host specificity will be 

 given later in this volimie in the section on 

 Evolution (p. 615). We conclude from the 

 evidence at hand that for given stages in 

 the life history, parasites, Uke free-living 



