18 THE ROCKY MOUNTAIN SPOTTED FEVER TICK. 
engorge, and, third, the existence of a certain amount of protection 
for the development of the stages when not on hosts, As a rule the 
abundance of ticks is dependent upon the amount of vegetation. 
Lands upon which some fallen timber and undergrowth occurs are 
usually found to harbor ticks in abundance, provided the hosts—cer- 
tain small mammals and domestic animals—are also present. In the 
Bitter Root Valley the areas in which more or less heavy second 
growth has followed the removal of the original timber have been 
found to be most heavily infested with ticks. These areas are locally 
known as “slashings.” (See PI. I, fig. 1.) 
It has been determined that the direct rays of the sun during the 
summer have a markedly injurious effect upon the early stages of the 
tick. This fact may be utilized to some extent, as will be shown later, 
in the control of the species by clearing the land of timber and under- 
brush. In small experiments it has been found that when the seed 
ticks are exposed to the sun during very hot weather they imme- 
diately crawl down the grass to the surface of the soil to seek pro- 
tection, and in the absence of an abundance of moisture death results 
in a very few days. The exposure of freshly deposited eggs to the 
sun at Dallas, Tex., has been found to cause them to shrivel and dry 
within less than a day’s time. 
The relative abundance of rain, especially during the spring 
months, in different years has a marked effect upon the number of 
ticks occurring in a given locality. This factor is of little importance 
in the natural control of the adult stage of the tick, but is a potent 
factor in the destruction of the eggs and immature stages, particularly 
after the latter have become engorged and dropped from the animal. 
Several other natural means of control of minor importance are 
also operating to some extent to keep the species in check. In barn 
lots, chickens have been observed to destroy the females which drop 
to the ground after becoming filled with blood. Some wild birds are 
known to feed upon various species of ticks, and in one instance, at 
least, they have been observed to destroy the engorged females of the 
spotted-fever tick. Certain species of ants are also thought to be im- 
portant enemies of the pest, particularly when the ticks are in the 
‘immature stages. 
Owing to the fact that the Rocky Mountain spotted-fever tick is 
primarily a northern form, and therefore accustomed to severe cold, 
it is doubtful whether severe winters are of much importance in its 
destruction. This is particularly true where there is an abundance 
of protection provided by brush and litter on the ground, 
