1985] 
Cokendolpher & Francke — Fire ants 
93 
tion in North America of the four species studied are between 35° 
and 37° north latitude. Solenopsis aurea Wheeler, the desert fire 
ant, generally does not occur east of the 100th meridian, whereas 
Solenopsis geminata (Fabricius), the tropical fire ant, and Solenop- 
sis invicta Buren, the red imported fire ant, generally do not occur 
west of the 100th meridian. Solenopsis xy/oni McCook, the south- 
ern fire ant, occurs across the U. S. from coast to coast (Francke et 
a /., 1983). Thus, we sought correlations among the species’ tempera- 
ture preferences in the laboratory and their observed ecogeographic 
distributions. 
Materials and Methods 
The position to which an insect will move in a thermal gradient 
indicates its preferred temperature. Orientation and movement in 
the gradient are stimulated by reactions of avoidance, attraction, or 
a combination (Deal, 1941). Extensive literature exists on preferred 
temperatures of terrestrial arthropods (see Bull and Mitchell, 1972; 
Fraenkel and Gunn, 1961; Mori, 1961; Protomastro, 1973; Singh et 
at ., 1967; and citations contained therein, particularly Deal, 1941). 
Two types of devices are generally used in these investigations: 
alternate chambers with temperature ranges of only a few degrees, 
and a gradual gradient chamber (linear or circular). In either case 
the movement of individuals or the final distribution of a large 
number of insects is recorded as a measure of temperature 
preference. 
Temperature is not the only variable in a temperature gradient 
apparatus. For a review of devices and associated problems, see 
Fraenkel and Gunn (1961), Madge (1965), and citations contained 
therein. In the present experiments, like Bull and Mitchell (1972), 
we attempted to reduce the effects of the non-temperature variables 
as identified by Madge (1961) by using: (a) a porous false floor, so 
that the substrate temperature would not differ significantly from 
the air temperature immediately surrounding it; (b) a large gradient 
chamber to promote microclimatic stability; and (c) atmospheres as 
near 0% and 100% R. H. as possible to minimize the effects of 
moisture gradients on the ants’ preferences. 
A linear temperature gradient apparatus, slightly modified from 
that of Bull and Mitchell (1972), was used for the studies (Fig. 1). 
The lid of the chamber was fitted with three magnets (one on top, 
