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Psyche 
[Vol. 88 
5 
o 
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DATE 
Figure 3. Daily occurrence of emigrations for Pheidole hvatti. Black bars 
indicate the number of colonies which emigrated each night. Upper line outlines the 
number colonies included in the sample each night. Right ordinate indicates rainfall 
for the superimposed bar diagram showing daily rainfall. 
the first heavy rainfall the soil loosened considerably. This un- 
doubtedly made the excavation of new nests by the desert-grassland 
dwelling colonies much easier. The same reasoning can be applied to 
explain the positive correlation between emigration frequency and 
daily rainfall in both habitats. However, in the oak-juniper wood- 
land, the greater amount of vegetation, the rockier soil and the 
generally moister conditions probably account for the relatively 
higher emigration activity before the beginning of the rainy season 
in this habitat. 
The need to perform a colony emigration is a contingency almost 
all species of ants can be expected to face (Wilson 1971). However 
some species emigrate more than others. Among the legionary ants, 
particularly the Ecitoninae and Dorylinae, colony emigrations are 
an integral part of the foraging ecology (Wilson 1971). Oppor- 
tunistic nesters such as Tapinoma melanocephalum, T. sessile, 
Paratrechina bourbonica and P. longicornis occupy ready-made 
nests such as the tufts of dead grass and hollow plant stems which 
