1986] Rissing, Johnson, & Pollock — Acromyrmex 181 
Table 1. Pleometrosis in Acromyrmex versicolor. 
No. queens in nest 
Collection frequency 
1 
28 
2 
16 
3 
6 
4 
7 
5 
3 
7 
2 
9 
1 
16 
1 
in shaded soil (Pontin 1960). Queens of the tropical leaf-cutting ant 
Atta cephalotes appear capable of choosing between major habitat 
types (mature evergreen woodlands as opposed to deciduous forest 
or cultivated fields) (Rockwood 1973). Similar ability to choose 
between major habitat types (woods versus open fields) while flying 
occurs in Lasius neoniger and Solenopsis molesta (Wilson and Hunt 
1966). 
Preference by queens for the canopy edge (as opposed to any- 
where under a tree) may represent a trade-off for shade while still 
being as warm as possible for rapid development of an initial worker 
force and eventual establishment of a foraging territory. This would 
be consistent with the high degree of pleometrosis in this species (see 
below) and with the “maxi-therm” hypothesis of Hamilton (1973). 
Location of a starting nest under a tree canopy (especially O. tesota 
whose branches frequently droop to the ground) would permit easy 
and safe access to vegetation for initiation and growth of the fungus 
garden characteristic of all leaf-cutters. Unlike most species of higher 
ants, queens of Acromyrmex spp., including A. versicolor, routinely 
forage for vegetation, especially at colony initiation (Weber 1972, 
Gamboa 1974). Trees, including O. tesota, are commonly harvested 
by A. versicolor (Gamboa 1975). 
Of the several hundred adult colonies of A. versicolor we have 
observed within the vicinity of Phoenix, AZ, virtually all have been 
located directly under adult trees and never in the open between 
trees. Acromyrmex, Atta and related genera are a largely tropical, 
New World group of ants; A. versicolor is the northernmost of 24 
Acromyrmex species and certainly one of the most desert-adapted 
of all the leaf-cutters (Creighton 1950, Weber 1972). Habitat choice 
by founding queens and location of adult nests under trees may be 
