1981] Mac Kay — Nest Phenologies of Pogonomyrmex 
35 
moisture and temperature. This probably accounts for much of the 
brood being kept in the lower nests levels. 
The harvester ants apparently obtain water from several sources. 
Some metabolic water may be available to the ants, as it has been 
shown that harvester ants increase their metabolism when they are 
water stressed, without increasing their activity (Ettershank and 
Whitford 1973; Kay and Whitford 1975). Morning dew would not 
normally be available as foraging begins after dew has evaporated. I 
have seen harvester ants actively drink rain drops on the soil 
surface, demonstrating a curious pumping action of the gaster, but 
precipitation is not common in the three habitats during the summer 
(U.S. Weather Bureau Climatological Data). Capillary condensa- 
tion occurs in the soil at relative humidities above eighty percent 
(Rode 1955) and may allow the ants free water. Arthropods, 
especially insects, are able to actively absorb water vapor from 
unsaturated air, although the mechanism is not understood (Edney 
1974; Cloudsley-Thompson 1975). It is not known if harvester ants 
have the ability to actively absorb water vapor. 
Seasonal changes in nest populations. 
The data on nest populations obtained from the nest excavations 
are summarized in Appendix 1. Absolute counts could not be easily 
compared because the numbers of individuals present in the nests of 
the three species are very different. To reduce this variation between 
nest populations of the three species, the data are compared in the 
form of percentages. The seasonal changes in the brood and sexual 
populations are similar for all three species, when the percentage 
composition of each of the classes are compared (Figs. 4 & 5). In the 
three species, egg laying begins in late April to late May, similar to 
P. owyheei (Willard and Crowell 1965) and P. occidentalis (Lavigne 
1969). Development from egg to callow in the species requires five 
to six weeks compared to 25 days for P. badius (Gentry 1974) and 30 
days in P. occidentalis (Cole 1934). It is very difficult to determine 
the number of larval instars in the development of ants (Wheeler 
and Wheeler 1976), although Marcus (1953) suggests that there are 
four instars in P. marcusi. As a consequence, all of the instars were 
combined into a single group. The first larvae appear about a week 
after the eggs are laid, first pupae about two weeks later. Callows 
are found in the nest about 5 or 6 weeks after the eggs were laid and 
