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Trigonoscelis gigas Reitt. (Coleoptera: Tenebrionidae) is introduced as a model system for studies of circadian rhythms as applied to ecology and space biology. Field observations are complemented with experiments on free-running in the laboratory. In the natural environment (Kara-Kum sand desert, Turkmenistan) the beetle population demonstrates 2 narrow (2 hr each) peaks of activity, a morning and an evening one. In DD, among 107 records, a diversity of free-running patterns was observed.

The long-term dynamics of free-running rhythms in DD reveals interaction between peaks, spontaneous alterations of tau, and splitting. Light and temperature pulses can change circadian phase phi or period tau, cause damping or re-appearance of rhythmicity. A surprisingly strong individual variability of basic features of the circadian system of T. gigas is emphasized.