116 Book-lice and Bark-lice; Biting Bird-lice 
The most interesting thing in connection with the Mallophaga, excepting 
their parasitic life and strange food-habits, is the puzzling problem of their 
distribution. The problem in its largest phase is this: the species of Mal¬ 
lophaga are, in a majority of cases, peculiar (so far as recorded) each to some 
one host species. But the instances are many where a single parasite species 
is common to a few or even to many host species. How does this condition 
of commonness to several hosts come to exist? 
As the Mallophaga are wingless, their power of migration from bird to 
bird is limited. Moreover, they can live for but a short time off the body 
of a warm-blooded host. After a bird is 
shot, the Mallophaga on it die in from 
two hours to three or four days: in rare 
cases living individuals are found on the 
drying bird-skin after a week. Although 
the parasites in a badly infested hen-house 
will be seen on the roosts and in the nests, 
in Nature the insects are rarely found off 
the host’s body. On such a likely place 
as an ocean rock from which I had just 
frightened hundreds of perching pelicans, 
cormorants, and gulls no parasites could 
be found. Practically migration must be 
accomplished while the bodies of the hosts 
are in contact. Such cases occur during 
mating and nesting, and when gregarious 
an albatross. (Greatly magnified.) birds roost or perch closely together. 
Occasional migration might occur from a bird of prey to its captured 
victim, or from victim to hawk. 
The general character of the cases in which a single Mallophagan species 
is common to several host-species may now be considered. Docophorus 
lari has been found on thirteen species of sea-gulls, and Nirmus lineolatus 
on nine. Gulls are gregarious, perching together on large food-masses 
and on ocean rocks. But on these rocks gulls are closely associated with 
other coast birds, as cormorants, pelicans, murres, etc. And the gull-para¬ 
sites might have opportunities to migrate to these other bird-species. 
Docophorus icier odes and T rinoton luridum are common to many duck species 
(each has been collected from nine), but ducks also are gregarious, and in 
addition are much given to hybridizing. But a parasite may be common to 
several host-species of non-gregarious habits. Docophorus platystomus is 
common to several hawk-species, D. cursor to several owl-species, D. excisus 
to several swallows, D. calijorniensis to several woodpeckers, and D. com¬ 
munis to several passerine birds. In the other genera of Mallophaga are 
Fig. 145.—Bonnet-shaped pharyngeal 
sclerite, lateral aspect, from a biting 
bird-louse, Eurymopetus taurus, from 
