26 THE ROCKY MOUNTAIN SPOTTED FEVER TICK. 
It has been observed that even though the adults which transform 
from overwintered nymphs are kept confined with the host animal 
during the summer or fall following their maturity, they show no 
marked desire to feed, usually endeavoring to crawl away and be- 
come quiet. Thus the habit of the adults of attaching to hosts in the 
spring appears to be so well established that they can scarcely be 
induced to attach to a host after midsummer. 
From the foregoing statements it is evident that although a few of 
the ticks may complete their life cycle—that is, the transformation 
from unengorged adults to unengorged adults of the next generation, 
or from unengorged nymphs to unengorged nymphs of the next gen- 
eration—during one season, the majority require two years for this 
eycle. Should overwintered nymphs not find hosts until late in the 
season and thus not become adult until the approach of winter, the 
resulting adults, if unable to find hosts, may survive until the second 
spring following. Ticks which pass the winter in the adult stage 
may survive until the second spring following, then engorge and pro- 
duce offspring which develop to nymphs the second summer, pass the 
winter in the nymphal stage, and complete development to unen- 
gorged adults during the third season. Thus it is apparent that 
under certain conditions three years might be required for the com- 
pletion of the life cycle. This would necessitate the destruction of 
the adult ticks during three successive seasons in order to eradicate 
the species. 
Figure 2 shows several of the ways in which development may 
proceed. 
THE HOST ANIMALS OF THE SPOTTED-FEVER TICK. 
The investigations conducted by Dr. Ricketts indicated that the 
Rocky Mountain spotted fever tick is restricted in regard to its 
host relations. Our investigation has shown that this restriction 
of certain stages of the tick to certain classes of animals is very well 
marked. Zhe examination during three seasons of nearly 800 wild 
mammals which are inhabitants of the Bitter Root Valley and numer- 
ous observations made elsewhere have shown that, with few excep- 
tions, only the immature stages of the tick are to be found on this 
class of hosts. On the other hand, the large domestic animals are 
the principal hosts of the adult ticks, and the immature stages are 
rarely, if ever, found upon them. This restriction of the adult stage 
to the larger mammals, now a firmly fixed habit of the tick, undoubt- 
edly arose from the fact that the adult ticks are so large that they 
can be easily removed by the smaller mammals. As will be pointed 
out in the discussion of remedial measures, this habit of the Rocky 
Mountain spotted-fever tick may be taken advantage of in the con- 
trol or eradication of the species. 
