FEATHER LICE I49 



examples of which are found on all the British orders of birds. These 

 genera do not differ greatly from each other and for the present are 

 contained in the one family. 



The Ischnocera, as we have already seen, are more specialised and 

 adapted to particular environments, and hence show a greater diversity 

 in their structure (Plates XXI and XXIII, a-c). This fact is reflected 

 in their classification by the larger number of families and genera into 

 which they are divided. The species found on British birds are contained 

 in forty-three genera. No attempt will be made to give the characters 

 of the famiHes and genera, which is a detailed and specialised subject 

 outside the scope of this book. The large number and anonymous state 

 of the Mallophaga make it impossible to do more than mention some 

 of the more interesting ones found on British birds. 



Passeriformes 

 Genera of Mallophaga recorded in Britain: Colpocephalunij Myrsidea, 

 MenacanthuSj Ricinus (Amblycera); Briielia, Sturnidoecus, Penenmnns, 



Philopterus (Ischnocera). 



In this country the passerine birds may be parasitised by species of 

 any of these eight genera. Five others have been recorded from this 

 order in the New World. The rook harbours five of these genera, which 

 in Britain is the maximum for any one species of passerine, but they are 

 not necessarily all found together on one individual. These lice illustrate 

 the rather curious fact that the size of the birds in an order has no 

 bearing on the number of different genera which may be found upon 

 them. The Passeriformes have thirteen genera, the Struthioniformes 

 (ostriches) only one. The passerine birds also illustrate another un- 

 expected fact, namely that the genera containing the largest lice are not 

 necessarily found on the largest hosts, despite the fact that there is often 

 a correlation between the size of host and louse within a given genus. 

 In Ricinus (Plate XXIII), a genus confined to the Passeriformes, the 

 largest females may measure 4.5 mm. (about one-fifth of an inch) in 

 length. Feather lice of a comparable size are found, amongst British 

 birds, only on hawks, ducks, and fulmars, all of which are considerably 

 larger than the robin and the finches which are the most usual hosts of 

 Ricinus in tliis country. 



