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Psyche 
[Vol. 88 
Results 
Colonies of both P. desertorum and P. hyatti showed consider- 
able variation in their frequencies of emigration (see Tables 1 and 2). 
One colony of each species (D-Jnll-2 and H-Jnl7-1) did not 
emigrate at all, while one P. desertorum colony (D-Jn20-1) emi- 
grated 8 times, and one P. hyatti colony (H-Jnl4-2) emigrated 16 
times. Despite this variability, both species showed a clear tendency 
to emigrate frequently: over one-half of the P. desertorum colonies 
emigrated at least 4 times, and over one half of the P. hyatti colonies 
emigrated at least 6 times. To statistically compare the emigration 
frequency of the two species, the percentage days for which an 
emigration was documented was calculated for each colony, and the 
percentages for each species were compared using the Wilcoxon 
two-sample test (Sokal and Rohlf 1969). No significant difference 
was found in the emigration frequency between the two species 
(.1 > P > .05). 
This similarity between species in emigration frequency can also 
be seen if the frequency of time interval between emigrations is 
compared. Figures 1 and 2 show the frequency of emigration 
interval for P. hyatti and P. desertorum, respectively. Both distribu- 
tions are strongly skewed to the right with a surprisingly high 
number of emigrations occurring 1 to 2 days after the previous 
emigration. No significant difference was found in the frequency 
distribution between the two species (Wilcoxon two-sample test: 
.4 > P > .2) 
The daily occurrence of emigrations among all colonies is shown 
in the form of frequency histograms in Figs. 3, 4 and 5. The upper 
line in the graphs outlines the number of colonies which were 
included in the sample size each night. Excluded from the sample 
were colonies which were raided by army ants, or were still suffering 
from the effects of an army ant raid (see Discussion). Superimposed 
over the graphs is a bar diagram showing the daily rainfall. 
A positive correlation was found to exist between emigration 
frequency and daily rainfall in all three cases (Spearman rank 
correlation coefficient: P. hyatti : r s = .28, N = 66; P. desertorum in 
oak-juniper woodland : r s = .25, N = 64; P. desertorum in desert- 
grassland : r s = .32, N = 70). The effect of rainfall on emigration 
frequency is most clearly seen in P. desertorum in the desert- 
grassland habitat (Fig. 5). During the 29 days before the first heavy 
