96 LIFE HISTORY OF NORTH AMERICAN TICKS. 
large mammals, but attaches to the heads of fowls and hares, upon 
which the first molt is passed. Following the second molt, which 
takes place off the host, it attaches to almost any of the warm- 
blooded animals. 
Mammals serve as the principal hosts of the ticks. Fowls are 
largely the hosts of the genera Argas and Ceratixodes, of two North 
American species of the genus Hemaphysalis, and of one of Ixodes. 
Several species of the genera Ixodes, Amblyomma, Aponomma, and 
Hyalomma are also occasionally parasitic upon fowls, and in the 
immature stages a number of others may attach to fowls. A large 
number of species have been collected from reptiles, several from 
amphibians, and two from beetles. 
ADAPTATIONS. 
Natural selection appears to have resulted in special adaptations 
both of function and structure and of the habits of ticks. All ticks 
must find hosts and attach at least once, some as many as four 
times, before they can reproduce. This necessity has resulted in 
special adaptations of function and structure for attachment. 
The adaptation of function is shown in the use made by the ixodid 
ticks of the front pair of legs. As one approaches the free tick, these 
legs can be seen waving in the air, while with the others it holds 
to its support. When a host comes in contact with them, they 
cling to it most tenaciously with these legs, to determine which 
fact one has but to pass a finger rapidly over a cluster of the seed 
ticks. An examination shows the legs to be especially fitted for 
attachment. 
The adaptation of structure for protection is represented in the 
case of the engorged larve of Argas mimatus. Up to within a few 
hours of dropping, these larve are globular in shape; but at this 
time they flatten and assume the typical Argas shape. This flat- 
tened form, common to all of the other stages, permits the ticks to 
crawl rapidly and to secrete themselves in cracks and crevices pro- 
tected from the wily fowl. In the Ixodine we find what may be 
considered specially adapted mouthparts, which, being unusually 
long, penetrate deeply and prevent their easy removal. 7 
In the adaptation of habits favorable to attachment and protec- 
tion we find most striking illustrations of natural selection. Such 
adaptations are: First, in molting; second, in attachment to any 
host; third, to habits of host; and, fourth, acquired greater vitality. 
There is a great disadvantage in dropping to molt, for it necessitates 
long periods of waiting, and results in a high percentage of mor- 
tality from not finding a host. This disadvantage is overcome by 
some species which have acquired the habit of molting on the host, for 
example, by Margaropus and by Dermacentor nitens and D, albipictus. 
