RECKONING TEMPERATURE. 35 
ton, Mont., late in December and in the first part of January from 
horses which had been in the hills during the winter. This species 
is undoubtedly Dermacentor albipictus, which has been found during 
our investigation to be present on horses and cattle in large numbers 
in both the nymphal and adult stages throughout the winter months. 
Thus it appears that even in the Boreal Zone some species are active 
during the winter months. When the season is not too hot and dry 
the ticks are most numerous during late summer and early fall. In 
the case of most of our species, we have found that all stages may 
occur on hosts at the same time of the year, there being no well- 
defined restriction of certain stages to any one season. 
In our work with Dermacentor venustus we found it almost impos- 
sible to get adults to attach to hosts durmg the summer months. 
They remain quiet for long periods with their legs curled up close to 
the body. This, together with the fact that very few adults are seen 
on hosts in nature after the middle of June, seems to indicate that 
there is a period of estivation during the latter part of the summer. 
Mayer has considered it possible that the cattle tick, which must 
find but one host in order to complete its cycle of development, may, 
where hosts are plentiful, pass as many as four generations in a single 
year. As the number of generations depends entirely upon the find- 
ing of hosts, it is impossible to say what the average annual number 
of generations would be. With the 3-host species, on the average 
probably not more than one generation is passed in a year, and in 
many cases a period of two years or even longer may be required for 
a single generation. - 
METHODS EMPLOYED IN STUDIES OF TICKS. 
RECKONING TEMPERATURE. 
Although it is now generally understood that the temperature above 
which active metabolism takes place in insects, ticks, and other cold- 
blooded animals may vary with each species or even with each stage 
in the development of a single species, we have not attempted, in 
preparing this bulletin, to determine such temperatures, but have 
thought it best to use 43°F. as the zero of effective temperature in 
all our computations. However, from the data furnished with each 
table one can work out the approximate effective temperature for 
a given stage. In figuring the total effective temperature required 
for the incubation of eggs we have commenced with the calendar day 
on which the eggs were deposited and included the day on which 
hatching occurred. In computing the total effective temperature 
required for molting we have commenced with the day following 
dropping and included the day upon which molting took place. The 
above periods have been used in determining the mean tempera- 
