30 
Psyche 
[March 
THE THATCHING ANT, FORMICA OBSCURIPES 
FOREL 
By A. C. Cole, Jr. 
Ohio State University 
Formica ohscuripes is apparently one of the most suc- 
cessful ants present in semi-arid regions of the West. Its 
mounds, composed of dried plant material, are successfully 
able to resist the strong, westerly winds common to that 
section of the United States. 
Wheeler 1 records this species from Wyoming, Montana, 
Colorado, Arizona and British Columbia. In addition to 
these localities I have found it in Idaho, Oregon and South 
Dakota. Wheeler 1 states that colonies of ohscuripes are 
most abundant at higher altitudes, from 5,000 to 8,000 
feet. Furthermore, the mounds are apparently most abun- 
dant in semi-arid localities, especially where sagebrush and 
its allied flora are growing. 
The mounds consistently are composed of twigs and 
other dry plant material and are more or less dome- 
shaped (fig. 1). Almost invariably they are centered 
around a dead sagebrush plant. Parallel with the stem of 
the sagebrush one or more galleries extend into the interior 
of the mound. At least one of these external orifices opens 
at the apical juncture of the mound with the sagebrush 
stem. Other entrances perforate the mound in widespread 
places. I have counted as many as 44 and as few as 3 
entrances on a single mound. 
From observations it is certain that the sagebrush is 
alive when the ants begin building the mound. When the 
colony possesses a suitable number of workers the bark is 
chewed from the base of the plant and formic acid is 
ejected on the tender cambium layer. Whether this formic 
