1981] 
Droual & Topoff — Genus Pheidole 
141 
EMIGRATION INTERVAL (DAYS) 
Figure 2. Frequency of the time interval between emigrations for Pheidole 
desertorum. 
emigration could be predicted in advance by the colony’s excavation 
activity at another site. For example, before colony D-Jnl2-2 
emigrated to its second nest site on 8/17, workers from the colony 
were observed excavating at the site on 8/4, 8/ 5, 8/7 and 8/ 10-8/ 17. 
However two colonies of both P. desertorum and P. hyatti were 
observed excavating at sites to which they did not emigrate even 
though they emigrated later to other nests. 
Discussion 
In this paper we have shown that P. desertorum and P. hyatti 
emigrate frequently and that the emigration frequencies of the two 
species are similar. This similarity in emigration frequency becomes 
even more marked when it is taken into account that most of P. 
desertorum ’s emigrations in the desert-grassland occurred after the 
first rainfall. The sharp increase in emigration activity after the rain 
can possibly be explained by the affect of the rainfall upon the soil. 
Before the rains began the soil was very hard and compact, but after 
