1981] 
Droual & Topoff— Genus Pheidole 
147 
and those that invoke an ultimate cause (Wilson 1971). For 
example, it has been hypothesized that the ultimate cause of army 
ant emigrations is to prevent a local depletion of food resources 
(Wilson 1971). The proximate cause of these emigrations, at least 
among the Ecitoninae, was discovered to be recruitment to a new 
nest under periods of high colony arousal due to brood stimulation 
(Schneirla 1938). However, it has been recently shown that food 
supply may also be a proximate factor (Topoff and Mirenda 1980, 
Mirenda and Topoff 1980). The hypothesis we are advancing to 
explain the frequent emigrations of P. hyatti and P. desertorum 
concerns the ultimate cause of these emigrations although both the 
ultimate and proximate causes are the subject of further investiga- 
tion by us. 
Both P. desertorum and P. hyatti , which are small and lack 
potent stings, are easy prey for army ants of the genus Neivamyrmex. 
Mirenda et. al. (1980) found, in the same desert -grassland site 
employed in this study, that P. desertorum was the species most 
frequently raided by N. nigrescens. Our own observations also show 
that both P. desertorum and P. hyatti are heavily preyed upon by 
members of the genus Neivamyrmex (Tables 3 & 4). Some P. 
desertorum colonies were raided repeatedly by the same army ant 
colony which entered the statary phase in a nearby bivouac. On two 
occasions an army ant colony actually bivouacked in the evacuated 
nest of a P. desertorum colony. One P. hyatti colony was raided by 
two species of Neivamyrmex. Of these colonies only five appeared 
to be completely eliminated by the army ants. Part of the reason for 
Table 3. Observed army ant raids on colonies of P. hyatti. 
Colony 
Dates of Raids 
Species Raiding 
H-Jnl5-1 
7/7, 7/8* 
Neivamyrmex nigrescens 
H-Jnl5-2 
7/8 
N. nigrescens 
H-Jnl9-4 
7/28 
N. texanus 
H-Jn21-3 
8/15 
N. opacithorax 
H-Jn21-4 
8/17 
N. nigrescens 
H-Jn21-5 
8/15 
N. opacithorax 
8/18 
N. nigrescens 
H-Jn23-1 
8/15* 
N. opacithorax 
H-Jn26-1 
8/12 
N. nigrescens 
H-Jn27-1 
8/12* 
N. nigrescens 
Colony was not seen afterwards. 
