FEATHER LICE I43 



Primitive genera. In the superfamily Ischnocera there are some genera 

 or groups of genera, which have a primitive type of head (Plate XXIIIa) 

 and which have not become adapted to any particular habitat on the 

 bird nor to the feather structure characterising any particular group of 

 birds. Examples of these less specialised genera are found on most of 

 the orders, and naturally appear more closely related to each other than 

 to those genera which have become highly specialised. Such primitive 

 genera cannot, therefore, be used in the consideration of relationships 

 between birds. 



Secondary infestations. It is rare, as we have already seen (p. 135), for 

 a louse from one bird species to be transferred to another, and if this 

 does take place, the host specificity of the louse makes establishment on 

 the new host difficult or impossible. Have there been, nevertheless, cases 

 in the past where a louse has become established on a new and different 

 host and there developed into a new and different species ? The answer 

 is almost certainly in the affirmative. Secondary infestation may ex- 

 plain the presence of one peculiar louse species found on the British 

 skuas (Stercorariidae). The skuas are related to the gulls and belong to 

 the order Gharadriiformes. They are parasitised by species of lice 

 belonging to four genera found throughout the Gharadriiformes in- 

 cluding the gulls. In addition, three of the species of skuas which visit 

 this country are parasitised by a louse belonging to another entirely un- 

 related genus, which is otherwise peculiar to the petrels (order Pro- 

 cellariiformes) . This strange distribution can be explained by assuming 

 that lice from a petrel managed to transfer to a skua, became established 

 and gradually developed into a new species. Skuas in this country are 

 known to feed on the dead bodies of at least one petrel, the manx shear- 

 water, which suggests a possible way in which the original transfer 

 might have occurred. Because the event has presumably taken place in 

 relatively recent times it is still possible to deduce what has happened. 

 The discovery of one species of petrel louse on the skuas does not tempt 

 us to suggest a relationship between the skuas and the petrels — any more 

 than some future etymologist would suggest a relationship between 

 English and Tamil because the word "curry" occurs in both languages. 

 However, the petrels are further parasitised by a genus which is also 

 found on the gulls and throughout the Gharadriiformes. This group is 

 one of the head-lice genera, which tend to become specialised to a 

 particular host, and are unlikely to be relics of a universal distribution. 



