1985] 
Cokendolpher & Francke — Fire ants 
97 
Intraspecific analyses of temperature preferences indicated that 
relative humidity was highly significant (p < 0.001) for all species: 
in S. aurea F(l,50) = 19.6; S. geminata F(I,48) = 65.4; in summer 
collected S. invicta F( 1 ,57) = 38.3, and in winter collected ants 
F(l,65) = 99.2; and finally in S. xyloni F(l,43) = 16.4. The two way 
interaction (R. H. X acclimation) was significant only for S. gemi- 
nata [F(2,48) = 7.8, 0.01 > p > 0.001]. The temperature preference 
responses of the four species of fire ants at both 0% and 100% R. H. 
appear in Fig. 2. 
Acclimation was significant only with S. xyloni [F(2,43) = 4.2, 
0.05 > p > 0.01], and individuals of S. invicta collected during the 
winter [F(2, 65) = 15.4, p < 0.001] (Table 1). Solenopsis invicta col- 
lected during the summer months showed no acclimation effects 
[F(2,57) = 1.3, p >0.05]. 
Discussion 
The ability to react rapidly to temperature gradients by the four 
species of fire ants, and the effect of relative humidity on this 
response have obviously been shaped by strong selective pressures. 
In all intraspecific pair-wise (with respect to acclimation) compari- 
sons possible, ants at 0% R. H. had a lower temperature preference 
by about 3-4°C with respect to those at 100% R. H. Each experi- 
mental trial lasted a minimum of two hours, indicating that the ants 
can perceive and respond to the lower, but constant, relative humid- 
ity despite the superimposed temperature gradients in a relatively 
short period of time. This reaction time is not totally unexpected 
given that ants can dehydrate quite rapidly. Studies on humidity 
preferences of the same four species of fire ants, in the absence of 
superimposed temperature gradients (Potts et al. , 1984), revealed 
that broodless worker ants failed to make scorable choices in a 
humidity gradient, whereas brood-tending workers always preferred 
near-saturated relative humidities. When tested at 22° C, this 
response of negative hydrokinesis required up to 24 hours to 
become expressed. 
These ground-nesting ants build mounds to variable extents 
depending on soil texture and moisture levels. The soil profile has a 
thermal gradient which varies with daily and seasonal cycles of 
insolation. Relative humidity at any given depth in the ant tunnels is 
influenced by the soil moisture and the temperature at that level. 
Thus, by detecting temperature and humidity gradients in their nest 
