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Animals avoid temperatures that constrain foraging by restricting activity to specific times of the day or year. However, because temperature alters the availability of food resources, it is difficult to separate temperature-dependent effects on foraging and the occupation of temporal niches. By studying two congeneric, sympatric Myrmecia ants we isolated the effect of temperature and investigated whether temperature affects foraging schedules and causes the two ants to be active at distinct times of the day or year. We monitored foraging activity and identified the ants’ temperature tolerance in the laboratory by determining (1) critical thermal minima and maxima (CTmin and CTmax) and (2) the relationship between walking speed and temperature. Ants of Myrmecia croslandi were diurnal throughout the year, but ceased above-ground activity during winter. Surface temperature at the onset of foraging was 9.8-30.1 degrees C, while their laboratory CTmin and CTmax were 10.4 and 48.5 degrees C, respectively. Time of foraging onset was significantly influenced by surface temperature at time of sunrise and of onset. Ants of Myrmecia pyriformis were nocturnal throughout the year. Surface temperature at the onset of foraging was 5.4-26.2 degrees C, while their laboratory CTmin and CTmax were 8.2 and 41.6 degrees C, respectively. Time of foraging onset was not influenced by surface temperature, but solely by sunset time. We conclude that temperature determines the timing of foraging as well as the daily and seasonal foraging activity in M. croslandi, but has less obvious effects on M. pyriformis. In both species, CTmax was greater than temperatures at the natural foraging times.