42 
Psyche 
[Vol. 88 
workers and the nest queen, overwinter near the 40 cm level of the 
nest (Figures 6 and 9). In the early spring the soil temperatures are 
low (Figure 2) and the ants are very sluggish. When the snow begins 
to melt, the lowest chambers of the nest fill with water. If the ants 
were at the lowest levels, they would probably be killed. In April 
and May the P. montanus worker population begins to spread 
throughout the nest. In June, July, and August, nearly 80% of the 
worker population moves into the upper 10 cm of the nest (Figure 
6). During this time the nest temperatures are high and much of the 
worker population is involved in foraging, brood care, and nest 
construction. In September as the soil temperature begins to cool, 
foraging decreases and the workers begin to spread throughout the 
levels of the nest. In December the workers are again at the 40 or 50 
cm level of the nest. The worker population in the 20 and 30 cm 
levels remains low and relatively constant throughout the year. 
There is apparently no temporal movement in the larvae or pupae, 
but they are present within the nest for only part of the year. In 
general, they are located at the 30 or 40 cm level where temperature 
and humidity are relatively constant throughout the season. The 
callows tend to occur in the deeper levels of the nest together with 
the brood. As most of the worker population is in the upper levels of 
the nest, the responsibilities of brood care are left to the callows. 
It is difficult to make inferences concerning the sexuals as 
individuals begin to leave the nest in the middle of August. Thus, 
what appears to be a downward movement may simply be the result 
of the individuals in the upper levels leaving the nest. The females do 
tend to occur deeper in the nest than do the males. They may be in 
lower levels in the nest in order to assist in caring for the brood, as 
has been observed in the laboratory. It has been shown in Formica 
polyctena that workers must learn brood care during an early period 
of their lives or they will never care for brood (Jaisson 1975). This 
could occur in Pogonomyrmex where the female reproductives may 
“learn” brood care so they can later rear their own brood. 
The seasonal movement in P. subnitidus nests is similar to that 
found in P. montanus nests (Figure 7). A high proportion of the 
workers remains in the upper 30 cm of the nest. In October there is a 
dispersion throughout the nest. By December, much of the popula- 
tion is at the 120 to 180 cm level, with little of the population in the 
lowest parts of the nest. The study area receives less snow than the 
