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1. We characterized year-round microclimate conditions (temperature and humidity) in and around necrotic cacti of the Sonoran Desert of southwestern North America. Necrotic cacti serve as host plants for four endemic species of Drosophila.

2. Flies in the field were exposed to high and variable temperatures, sometimes ranging between < 5degreesC to > 40degreesC in a single 24-h period. Habitat temperatures often exceeded thermal tolerance limits measured in the laboratory.

3. The air inside necroses was more humid than the air outside, but was often warmer during the day than outside air. Thus, air pockets within necroses do not provide a thermal refuge.

4. Necrotic tissues inhabited by Drosophila larvae reached temperatures in exeess of 40degreesC, as did the moist soil in which one of the endemic Drosophila species undergoes larval and pupal development.

5. We conclude that high temperatures provide a significant environmental stress for Sonoran Desert Drosophila, at all developmental stages. (C) 2003 Elsevier Science Ltd. All rights reserved.