In all animals, the aging process is accompanied by physiological and behavioral changes. Physiological changes, such as those associated with the nervous and musculoskeletal systems, often result in decreased or impaired locomotion in aged individuals (Fernandez et al., 1999; Hilleras et al., 1999; Le Bourg, 1987; Minois et al., 2001; Scimonelli et al., 1999; Siwak et al., 2002). Age-related locomotor deficits are often the result of degeneration in the central and/or peripheral nervous systems. In mammals, loss of neurons and neurotransmitters in cortex and cerebellum can be associated with aging and motor impairment (Hilber and Caston, 2001; Kaasinen et al., 2000; Volkow et al., 1998). Sensory information from the limbs is important in postural control and reactions to perturbations (Maki and McIlroy, 1996; McIlroy and Maki, 1996). Degeneration in the peripheral nervous system could result in loss of vibration or cutaneous sense in the limbs and a decrease in the response time of reflexes (Shumway-Cook and Woollacott, 2000; Ulfhake et al., 2002). Locomotor deficits in aged individuals can also be caused by non-neural degeneration in the musculoskeletal system. Aging is frequently associated with weakness and decreased mass in the muscles (Anderson, 2003). Muscle weakness in the elderly has been attributed to a reduction in the sliding speed of cross-bridging elements (Hook et al., 2001; Lowe et al., 2002). Muscle atrophy with aging is the result of a loss in the number of muscle fibers, rather than a reduction in fiber size (Lexell, 1993). Stiffness at the joints and ligaments, which can be caused by increases in collagen fiber cross-linkages and loss of elastic fibers, can also result in a decrease in the range of motion in aged individuals (Aigner and McKenna, 2002; Kerin et al., 2002). Insects provide a useful model system for aging studies because they are short-lived compared with mammals. Decreases in spontaneous activity with increasing age have been well documented in fruitflies and honeybees (Fernandez et al., 1999; Minois et al., 2001; Parkes et al., 1999; Tofilski, 2000). However, none of these papers have examined changes 4453 The Journal of Experimental Biology 206, 4453-4465 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00714 Aging is often associated with locomotor deficits. Behavior in aged Blaberus discoidalis cockroaches was analyzed during horizontal walking, climbing, righting and inclined walking. Adult animals showed a decrease in spontaneous locomotion with increasing age. Tarsal abnormalities, termed ‘tarsus catch’, were often present in aged individuals. In ‘tarsus catch’, the prothoracic leg catches on the mesothoracic leg during the swing phase. This deficit causes alterations of the gait, but animals are able to regain a tripod gait after the perturbation. The tibio-tarsal joint angle in individuals with ‘tarsus catch’ was significantly less than in intact animals. Structural defects were consistently associated with ‘tarsus catch’. The tracheal tubes in the tarsus and around the tibio- tarsal joint were often discolored and the tarsal pads were hardened in aged cockroaches. All aged individuals were able to climb. However, prior to climbing, some animals with ‘tarsus catch’ failed to show postural changes that are normally seen in young animals. Aged individuals can right as rapidly as 1-week-old adults. However, animals with ‘tarsus catch’ take longer to right than aged intact individuals. Old cockroaches have difficulty climbing an incline of 45°, and leg slipping is extensive. Slipping may be caused by tarsal degeneration, but animals that are unsuccessful in inclined walking often show uncoordinated gaits during the attempt. Escape behavior was examined in aged American cockroaches (Periplaneta americana). They do not show normal escape. However, after decapitation, escape movements return, suggesting that degeneration in head ganglia may actually interfere with escape. These findings provide evidence for age-related changes both in the periphery and in the central nervous system of cockroaches and stress the importance of multi- level approaches to the study of locomotion. Movies available on-line. Key words: senescence, walking, arthropod, central nervous system, kinematics. Summary Introduction Effects of aging on behavior and leg kinematics during locomotion in two species of cockroach A. L. Ridgel*, R. E. Ritzmann and P. L. Schaefer Department of Biology, Case Western Reserve University, Cleveland, OH 44106, USA *Author for correspondence (e-mail: alr17@cwru.edu) Accepted 8 September 2003