388 Relations between Species 



again in the growth of the sporophyte of a moss upon the tissues of 

 the leafy gametophyte. In this chapter, however, we are concerned 

 primarily with the parasitism of one species on another species. 



Great variety is displayed among parasites in regard to their loca- 

 tion on their hosts, the duration of their dependency, their adapta- 

 tions, and their methods of transmission. Only brief mention of the 

 more ecological aspects of this large subject can be made here. For 

 a more thorough discussion of parasitism the reader should consult 

 such authors as Pearse (1942), Chandler (1944), or Baer (1951). 

 Parasites may occur on the outside of the hosts (ectoparasites) or 

 within their cavities or tissues (endoparasites). Certain worms and 

 many bacteria and protozoans are obligate parasites practically 

 throughout their lives; other organisms may be parasitic for only a 

 small part of their existence. Facultative parasitism occurs widely 

 and is found, for example, in mosquitoes that can live their whole 

 lives on nectar and other plant exudates but will readily parasitize 

 any available mammal. However, some species of mosquito must 

 obtain a blood meal before they can reproduce. 



Many part-time parasites are parasitic as adults and free living as 

 larvae, but in other species the reverse is true. A rather special con- 

 dition within the latter category is presented by the parasitic wasps 

 that lay their eggs in the bodies of other insects. When the eggs 

 hatch, the larvae eat the tissue of the host, which at first remains 

 active. Since the host is always killed eventually by the growing 

 stowaway, the term parasitoid is sometimes applied to insects follow- 

 ing this life pattern. Certain of the true wasps complicate the pro- 

 cedure further by stinging their victim into permanent paralysis be- 

 fore depositing the egg on it. In this way, the parent wasp provides 

 a living but helpless insect of another species, or a spider, for the 

 nutrition of the larva whose mode of life is thus transitional between 

 parasitism and predation. Perhaps the most extraordinary example 

 of this behavior is the attack of digger wasps, the largest of which 

 have a wing span of about 10 cm, upon the even larger tarantulas. 

 When the female of the giant wasp Pepsis marginata is ready for egg 

 laying, she somehow locates a tarantula Cri/toplioUs portoricae, and 

 explores it with her antennae to make sure that it is the correct species. 

 The larvae of each species of wasp can be nourished by only one 

 species of tarantula. Although the tarantula could easily kill the 

 wasp, it does not do so, and makes little attempt to escape. After the 

 wasp has dug a grave for its intended victim, she stings it (Fig. 10.11), 

 drags it into the grave, and lays a single egg, which she attaches to 

 the abdomen of the paralyzed monster. At hatching the wasp larva 



