112 
Psyche 
[Vol. 95 
Figure 2. Thatch mound of Formica obscuripes. Carbon Co. 15 mi south of 
Bridger. The card measures 3 by 5 inches. 
the grasslands. A typical mound is conoidal or paraboloidal, 24 
inches (60 cm) in basal diameter and 5 l A inches (14 cm) high. It is 
composed of excavated soil and covered with a layer of fine gravel 
collected by the workers from the surface of the surrounding soil. A 
mound is rendered more conspicuous by a circular bare area which 
surrounds it. These areas average 5 ft (1.5 m) in diameter. The 
mounds of P. owyheei are similar. 
obligatory slave-makers. Obligatory slave-makers are inca- 
pable of performing any of the nest-functions and are therefore 
wholly dependent upon their slaves. The story of how they raid the 
nest of some species of Formica to get their slaves is fascinating but 
too long for this essay. The Montana slave-maker was Polyergus 
breviceps and its slave was Formica subsericea. 
facultative slave-makers. These are in the genus Formica 
and they enslave other species of Formica. They are, however, ca- 
pable of performing all necessary nest functions; hence they can, and 
often do, get along without slaves. The species of this group that 
