The Niallophaga, a suborder of the Pthiraptera, are a group of obligate
ectoparasites living on birds and mammals which present interesting problems
of evolution and phylogeny.+ The present distribution of the avian Niallophaga
suggests that these insects became parasitic on the birds early in the
evolution of the latter class and that they evolved with their hosts. In a group of related host species, each species may have allopatric species of a number of sympatric genera of Niallophaga common to the host group (VI: tables
1+, 5, 6), and in addition, sympatric species of one or more of these genera.
In many cases, therefore, a single host species may have a considerable number
of genera and species of Mallophaga (1 :279). The problem is to find an explanation of the presence of often closely related genera and species in
what is the equivalent of a restricted and isolated geographical area. A
study of the morphology of the Mallophaga and their present distribution both
on a single host individual and throughout the class Aves makes it possible to
deduce some of the factors which may have influenced speciation in this group
of ectoparasites and to compare these factors with those influencing groups of
free -living animals.
Apart from the intrinsic interest of evolutionary problems in a group of
ectoparasites, it is necessary when attempting to formulate a natural classification of the Mallophaga to have some understanding of the possible steps in
the evolution of the group.