BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



257 



on occasion may depend for its fish food 

 upon the osprey (Pandion haliaetus). The 

 osprey is the better fisherman, and is victim- 

 ized by attack in the air when carrying a 

 captmed fish. Such habits develop at the 

 individual level, the bald eagle as a species 

 being by no means dependent upon the 

 osprey. The robber-victim relation has be- 

 come more fixed in various other birds, 

 notably in the frigate bird (Fregata), which 

 robs various sea birds, and in the marauding 

 gulls known as skuas and jaegers (Sterco- 

 rariinae), which force their weaker cousins 

 to disgorge already swallowed prey (Knowl- 

 ton, 1909). It appears that robbery is indi- 

 vidually habit-forming, since it is reported 

 as well known in the honeybee, in which 

 individual workers may take to robbing 

 neighboring hives. Such robber bees are re- 

 garded as a nuisance by bee keepers, and 

 are repelled and killed by the workers in 

 the invaded hive; the robbing habit seems 

 to be easily estabfished and, once estab- 

 lished, not likely to be lost, in spite of the 

 unnecessary life-and-death hazard to which 

 the individual robber bee is exposed (Root 

 et al., 1945). Various predaceous Diptera 

 in the tropics take up positions along the 

 fine of march of army ants and driver ants, 

 and rob the worker ants of their prey, and 

 they are joined by various passerine birds 

 that take ants as well as ant-booty (Be- 

 quaert, 1922). 



There is a difference between parasites 

 and predators in their relation to the pyra- 

 mid of numbers (see pp. 522, 523). The 

 typical predator-prey pyramid has a broad 

 base of key-industry forms and a restricted 

 apex of relatively large master carnivores. 

 Contrariwise, in the parasite-host pyramid, 

 with each step from the primary host, the 

 parasites become smaller and more numer- 

 ous. One rat may carry a population of a 

 few tens of fleas, each of which may sup- 

 port a great many herpetomonad flagellates. 



After considering all known differences, 

 we agree with Elton (1927, p. 75) that the 

 resemblances between parasite and predator 

 are more important than the differences. 

 From our point of view, successful parasit- 

 ism may be regarded as a compromise or 

 partial truce between two living popula- 

 tions; the truce may be broken and severe 

 injury result for either parasite or host 

 whenever conditions become especially fa- 

 vorable for one or the other (cf. Smith, 

 1934). 



The types of dichotomies illustrated by 

 parasites include the following: 



(c) Location on host: ectoparasitism or 

 endoparasitism. Numerous transitions from 

 ectoparasitism to endoparasitism are known, 

 such as those shown by series of small mites 

 that five on the skin of various mammals or 

 peneti-ate it in one way or another. Jigger 

 fleas contrasted with ordinary fleas illustrate 

 another such series, as also do barnacles on 

 whales; certain species of barnacles, appar- 

 ently originally merely epicoles, now five as 

 internal parasites below the surface of the 

 whale's skin. Consider also the robber-vic- 

 tim relation, in which the parasitism is be- 

 havioral. 



{h) Duration of parasitism: temporary or 

 permanent. Tapeworais and many other ani- 

 mals remain parasitic through practicaUy all 

 the stages of their fife cycle. In other in- 

 stances parasitism is limited to one stage in 

 the fife history. Often the larvae are free 

 swimming and serve as dissemules, as in the 

 crustacean Sacculina, parasitic on the 

 gonads and other tissues of certain crabs; 

 the taxonomic relationship, obscured in the 

 highly degenerate adult, is revealed by the 

 characters of the naupUus larva. Perhaps 

 less often, the larva is parasitic, as in the 

 Gordius, the fresh-water "horse-hair snake." 

 The larvae of various species of Gordius are 

 parasitic, and the adult is free-hving. This 

 is the characteristic situation among the 

 parasitoid insects and in the trombid mites. 

 Often there are compHcations. The common 

 fresh-water unionid mollusks release their 

 glochidia as free-swimming organisms when 

 a fish is near; a glochidium swims for a 

 brief period and, if lucky, becomes parasitic 

 for a time in the gills or fins of its host and 

 later, after metamorphosis, leaves it and 

 takes up a sedentary bottom-dwelHng exist- 

 ence. Many other variations are known. 



(c) Necessity: facultative as contrasted 

 with obligate parasitism. Crabs of the genus 

 Pinnotheres may live independently, but 

 both adult and larvae of P. littoralis, for ex- 

 ample, enter the mantle cavities of certain 

 marine mussels (Wells, 1940). At the other 

 extreme, many tapeworms are obligate para- 

 sites at all stages in their life history. 



(d) Specificity: The specificity of para- 

 site-host relationships is a complex subject. 

 One type of host specificity is shown when 

 the parasite transfers directly from one de- 

 finitive host to another of the same or re- 

 lated land without living in an intermediate 



