Diverse physiological features characterize the diapause state. Development is halted or dramatically retarded, the cell cycle is arrested, metabolic rates are suppressed, and a global metabolic shift from aerobic to anaerobic metabolism is evident. Energy reserves and body water are usually not replenished during diapause, thus conservation of these resources is essential. Patterns of heartbeat and discontinuous gas exchange are distinct during diapause. Structural modifications such as flight muscle degeneration in adults and cytoskeletal distinctions are evident at both tissue and transcriptomic levels. Defense responses are usually bolstered. Heat shock proteins are commonly upregulated, as are immune and antioxidant responses, as well as cold-hardening mechanisms and hypoxia responses essential for surviving in winter habitats that are oxygen-limited. Diapause is not static, as evidenced by systematic shifts in metabolism and energy sources tapped at different phases of diapause, as well as changes in responsiveness to exogenous hormones or environmental stress.