INTRODUCTION 17 
pediculids, mallophagids, hippoboscids, and sarcoptids. In other cases 
the same effect is attained by a different process, as the flattening of 
the body vertically in fleas and horizontally in most other permanent 
parasites. Modifications of the mouth-parts, eyes, and antennx are 
very great, and furnish most striking examples of the modification of 
structures for adaptation to special conditions. 
DISTRIBUTION OF PARASITES. 
With reference to all of the more strictly parasitic forms, we may say 
that each group of animals has its particular forms of parasites and that 
each species of bird or mammal may be said to have its particular 
parasite fauna, while each parasite has its special limit of host species. 
Considering only the forms affecting birds and mammals, we find the 
(stride confined to mammals, and of mammals the genus Dermatobia 
to Primates and Carnivora, Gastrophilus to Equidze, Hypoderma to 
ruminant ungulates, Gistrus to Ovid, and Cuterebra to rodents. 
Hippoboscide are confined to certain families of birds and mammals— 
Olfersia to raptores, Lipoptena alternates between birds of the grouse 
family and mammals of the deer family, Melophagus is confined to sheep, 
Hippobosea occurs on the horse, and Ornithomyia occurs on various 
birds. 
The Pediculide are all confined to mammals—Pediculus to man, 
Pedicinus to monkeys, Hematopinus to Carnivora, Ungulata, and 
Rodentia, Echinophthirius to Pinnipedia, Hematomyzus to the ele- 
phants, and Hematopinoides to Geomys. 
The Mallophaga are all confined to birds or mammals and with the 
exception of Trichodectes and Gyropus, which occur on mammals, all are 
confined to birds, Trichodectes affect Carnivora, Ungulata, and Roden- 
tia, while Gyropus is confined to the guinea pig. The genera confined 
to birds have a quite varied distribution among the different groups, 
the same genus frequently having representatives in widely different 
families of birds, while any one species of bird may harbor a number of 
Species in several different genera. Moreover, each species of Mallo- 
phaga favors some particular region of its host, certain species occur- 
ring on the wings, others on the breast and others on the neck and head. 
Phe Pulicide affect both birds and mammals, and while less strictly 
limited than the lice, have usually definite range of hosts. Sarcopsylia 
has one species affecting mammals and another affecting fowls. Pulex 
contains species affecting both birds and mammals. Typhlopsylla 
occurs mainly on Rodentia and Insectivora. 
The Sarcoptidee and Linguatulide, which are the only permanent 
parasites among the arachnids, have definite hosts and usually little 
range of host forms. The genera occurring on birds do not ordinarily 
contain spevres affecting mammals, and, on the other hand, the genera 
occurring nermally on nammals do not present avian parasites. The 
Linguatulide present a case of migratory parasites, the species being 
4653—No. 5 2 
