February, ’20] PARKER: DERMACENTOR VENUSTUS BANKS 
35 
the rodents which were found to be hosts of the larval and nymphal 
ticks, were found. Hence it would be supposed that this was the place 
the adult ticks would occur, and during the early spring months they 
were found in such places, in considerable numbers. In June, on the 
other hand, the ticks were scarce on the slopes, but abundant in the 
draw. At this time the slopes had become dry, but the draw at 
the bottom was moist, and the vegetation green and abundant. For 
example, in a certain coulee known as Wolf Den Coulee, which was 
about half a mile in length, the floor gradually narrowed from a width 
of about one hundred feet at its mouth, to a few feet at the upper end. 
On May 18, operations on the floor of the coulee failed to reveal any 
ticks, though they were found in abundance on the slopes at the blind 
end. On June 19, however, numerous ticks were “picked up” the 
whole length of the floor. In another coulee in which the draw was but 
from one to three feet wide, nearly two hundred ticks were secured in 
late June by dragging but a few hundred feet up the draw. Their 
absence here, and their abundance on the slopes above early in the 
season had been noted in previous operations. Similar conditions were 
encountered in widely separated localities. The improbability that 
the ticks had been dropped in these bottoms as engorged nymphs was 
shown by the fact that the hosts of the immature ticks inhabited not 
the draw, but the slopes above. The only reasonable explanation 
seemed to be that the ticks had gradually migrated downward, and 
concentrated in the bottoms. What the compelling factor was would 
be hard to say, though the possibility that they had migrated from the 
dry slopes to the moist bottoms was naturally suggested. The writer 
has met with similar movements in other localities however, when the 
moisture factor seemed to be absent. It is of interest to inquire as to 
what becomes of the ticks migrating to the bottoms. They were 
certainly not numerous there in the spring in which this work was 
conducted, but there is nothing to explain their absence, or to suggest 
that they might not have been numerous the following spring, except 
the fact that these draws are usually filled by rushing torrents in the 
early season, and very wet for some time thereafter. It is well known 
that this tick avoids wet places, and is not normally found there. 
In 1918, when the writer took up the control work in the Bitter Root 
Valley, a few experiments were conducted, to see if migration actually 
occurred. For one experiment, a slope to the north of Big Creek near 
Victor, was selected. A certain trail on this slope had long been known 
to be heavily infested, the ticks always being found on the vegetation 
growing along the upper edge of the trail. Even before going to the 
Bitter Root Valley, my observations noted above had suggested the 
possibility that ticks migrated down the slope, and stopped when they 
