244 



PSYCHE. 



Wherever actual contact occurs be- 

 tween the bodies of the hosts, however, 

 migration can and evidently does take 

 place. In this way is the parasite 

 species perpetuated on its normal host 

 species; the parasites can migrate from 

 male to female, and vice versa, during 

 copulation, and from parents to young 

 in the nesting season. But instances of 

 contact among other individuals of the 

 same host species and even among indi- 

 viduals of different host species are not 

 only possible but in the case of certain 

 birds are probably frequent. Wherever 

 such contact occurs no further explana- 

 tion of the occurrence of single parasite 

 species common to two or more host 

 species is necessary. There is also a 

 possible explanation of possible rare 

 cases of commonness of parasite species 

 to two or more host species whose indi- 

 viduals apparently never come in contact. 

 Sharp * has recorded a case in which a 

 Hippoboscid (winged Dipterous parasite 

 of birds) which was captured while fly- 

 ing " was found to have some Mallo- 

 phaga attached to it." Such instances 

 must be rare. 



It will be of interest now to note the 

 various kinds of, cases (for the cases are 

 fairly amenable to classifying) in which 

 a parasite species is common to two or 

 more host species. I have taken Doco- 

 phorus lari from thirteen species of gulls 

 (twelve species of Larus and one of 

 Rissa), and N^irmus lineolatus from nine 



* Sharp, D, , Proceedings 



1S9O, p. XX.X. 



logical society of Londo 



species of gulls (all Larus). The gulls 

 are gregarious in habit, roosting together 

 in great numbers on ocean rocks. Un- 

 doubtedly there is frequent absolute 

 contact of the bodies of individuals of 

 various species, giving opportunity for 

 actual migration. But together with the 

 gulls on the ocean rocks sit other mari- 

 time birds ; for example, on Scotland's 

 famous Bass Rock Sula bassana (a gan- 

 net), Aka troile (an auk) , and Rissa 

 tridactyla (a gull) breed socially together 

 (see the striking wall-case of Bass Rock 

 birds in the Natural History Museum, 

 Kensington, London) . On the " bird 

 rocks " off Monterey, California, I have 

 found cormorants, pelicans and gulls 

 roosting together, and have found Lipcu- 

 rtis foxoceras, a long known parasite of 

 the cormorants, on a pelican {Pelecanus 

 calijornicus) , as well as on the cormorants 

 (^Phalacrocorax dilophiis albociUatus) . 

 Actual migration of the parasites from 

 the captured prey to the raptorial cap- 

 turer may evidently take place ; I have 

 found a Physostomum (genus normally 

 limited to passerine birds) on a sparrow 

 hawk. It is conceivable that among 

 birds with whom hybridization is not 

 infrequent a parasite species may 

 become common to several hosts. 

 Thus the hybridizing which is notorious 

 among allied duck species may help 

 account for the fact that Docophonts 

 icterodes is common to many duck 

 species (I have taken it from nine) and 

 that Trinoton hiridum is similarly com- 

 mon to many hosts (I have taken it also 

 on nine duck species) . 



