- We studied the effect of air temperature and RH on the survival time of adult Xenopsylla conformis Wagner, 1903 and Xenopsylla ramesis Rothschild, 1904 fleas during starvation to explain the paratopic habitat distribution of these species on opposite ends of a precipitation and temperature gradient in the Negev Highlands, Israel. We hypothesized that the pattern of distribution of the two flea species is caused by differential microclimatic preferences of imagoes and predicted that (1) the resistance to starvation would differ between the two flea species at different air temperatures (10 degrees C - 38 degrees C) and relative humidities (RH) (40%-92%) and (2) survival time of starving X conformis would be longer than that of starving X ramesis at high air temperatures and low RHs. Survival time of newly emerged X conformis was dependent on air temperature but not on RH, whereas in newly emerged X ramesis it was affected by both air temperature and RH. Generally, survival time of newly emerged fleas was longer at lower air temperatures and higher humidities than at higher air temperatures and lower humidities. No significant difference in survival time between species in dependence on either air temperature or RH were found for newly emerged fleas. Fed fleas of both species responded similarly to changes in air temperature and RH in terms of survival time. Survival time at lower temperatures was longer than that at higher temperatures. Females survived longer than males at all air temperatures except for the highest temperature when the survival time of both sexes was similarly low. In both species, the effect of RH on survival time was significant at the highest RH only, with a decrease in survival time at 92% RH. In contrast, survival times at lower RHs did not differ. Males of both species survived for less time than females at all RHs. X conformis generally survived for less time than X ramesis, all else being equal. The only regime at which X conformis survived longer than X. ramesis was 38 degrees C and 40% RH. Newly emerged fleas survived for significantly longer time than fed fleas.