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First published online November 10, 2003

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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

View larger version (16K): [in a new window]   Fig. 1. Adult lifespan of cockroaches and spontaneous locomotion. (A) Blaberus discoidalis (N=73) and Periplaneta americana (N=31) cockroaches were placed in plastic containers immediately after molting to the adult stage. Containers were inspected weekly, and dead animals were counted and removed. Blaberus did not begin to die until week 36 post-adult molt, while Periplaneta showed a gradual dying-off. Periplaneta survived up to 64 weeks but Blaberus could live up to 80 weeks post-adult molt. 11% of the Blaberus discoidalis lived to 73 weeks after the adult molt. However, by week 61, 50% of the individuals were dead. (B) Spontaneous locomotion was defined as the total time spent walking, climbing or burrowing in a 10 min period. Spontaneous locomotion decreased significantly as adult cockroaches aged (regression analysis, slope=-3.4, r2=0.839, P<0.01).

View larger version (42K): [in a new window]   Fig. 2. `Tarsus catch' in aged animals. (A) Video image (from the side and below) of a 63-week-old adult with `tarsus catch'. Note that the right prothoracic tarsus catches on the tibia of the right mesothoracic leg. Inset: close-up of the tarsus catching on the mesothoracic leg. (B) `Tarsus catch' is not evident until 60 weeks post-adult molt, and the percentage of animals exhibiting this behavior increases up to 65 weeks. However, this percentage decreases in animals that live more than 65 weeks. (C) Gait pattern in an animal with `tarsus catch' in the right prothoracic leg. The swing phase is shown as filled boxes. Prior to `tarsus catch', this animal walks in an alternating tripod gait. The vertical line represents the time when the right prothoracic tarsus catches on the mesothoracic tibia. The ellipse illustrates the absence of swing in the left prothoracic and metathoracic leg due to `tarsus catch'. In addition, the length of the swing phase is reduced in the other legs to compensate for the absence of support by the left prothoracic leg. The animal recovers within one leg cycle.

View larger version (45K): [in a new window]   Fig. 3. Analysis of tibia placement in horizontal walking. (A) The end of the tibia (dark gray dots) and the center of mass (COM; white dot) were digitized using motion analysis software. These points were used to calculate the distance of the end of the tibia during the stance phase to the COM (r; white line) and the angle of the tibia relative to the COM (; angle of white line and black line). (B) Leg placement values were plotted in a polar graph (r, ) for three successive steps in an intact aged animal (60 weeks post-adult molt). The start of the arrow shows the anterior extreme position (AEP), and the arrowhead represents the posterior extreme position (PEP). (C) As a comparison, leg trajectories for steps of the same animal shown in B were plotted after `tarsus catch' was evident (week 63). This graph shows that tibia placement is altered slightly in trials with `tarsus catch'. (D) Leg placement values in a 1-week-old adult. (E) Summary polar plot. Values of r and during the AEP and PEP were averaged in the same-aged animals in trials before and after `tarsus catch' developed. AEP and PEP values of the left legs were converted to values between 0° and 180° for statistical analysis. Dotted lines between the AEP and PEP are only used to link the points within a single leg. Subtle differences between trials before and after `tarsus catch' has developed are present (see Table 1). However, these differences could not be the cause of `tarsus catch'.

View larger version (117K): [in a new window]   Fig. 4. Orientation of the tibio-tarsal joint in aged animals. (A) A 60-week-old intact animal. The arrow shows the tibio-tarsal joint of the right prothoracic leg. (B) The same animal as in A at 63 weeks with `tarsus catch'. The angle of the joint is decreased in the animals with `tarsus catch'.

View larger version (124K): [in a new window]   Fig. 5. Tarsus morphology in young and aged animals. (A) The tarsal pads of a 1-week-old adult are white in color (arrow), and the joints between the tarsal segments are flexible (not shown). (B) By contrast, the tarsal pads of an animal with `tarsus catch' are brown in color (arrow) and the joints are often stiff. (C) A nylon filament, producing a force of 29 mN, readily deformed the tarsal pads in young animals. (D) Pads of old individuals were hardened and were not deformed by the nylon filament. (E) The cuticle of tibia was removed to examine the internal morphology of these leg segments. The trachea and tendon (arrow) in the tarsal segments were healthy and silver in color. (F) The trachea and tendon in the tarsus of aged cockroaches are discolored, hardened and degenerated.

View larger version (63K): [in a new window]   Fig. 6. Block climbing in aged and young cockroaches. (A) Posture of aged animal when approaching a three-block obstacle. (B) After detecting the obstacle, animals often change their body posture by rotating the mesothoracic legs forward and `rearing up' the front of the body. (C) Some aged individuals do not alter their body angle and run into the side of the block ('head butt'). (D) Summary of climbing behaviors in aged and young cockroaches. In most of the trials, aged animals rear up before reaching the block (as seen in young animals). However, in a few trials, aged cockroaches with `tarsus catch' run into the side of the block before climbing over it.

View larger version (10K): [in a new window]   Fig. 7. Righting duration in aged versus young animals. Aged animals are readily able to right themselves, and righting duration is not significantly different from that recorded in 1-week-old adults. However, the variability in timing of righting is increased as animals increase in age.

View larger version (62K): [in a new window]   Fig. 8. Inclined walking in aged animals. (A) Animals were placed in a treadmill with an acetate belt that was tilted at a 45° angle. The analysis was started when the caudal end of the animal reached the beginning of the incline (START). (B) Trials were recorded as successful when the animal walked up the incline approximately one body length from the START point (END). (C) Leg slipping, defined as a change in the position of the foot on the substrate during the stance phase, was often present during inclined walking in aged animals. The probability of slipping was calculated as the number of slips per step for each animal. Probabilities for each individual were averaged across the population. There were no differences in the amount of leg slipping between animals with and without `tarsus catch'. However, prothoracic legs slipped more often than the mesothoracic or metathoracic legs. (D) Data from the inclined walking trials were reorganized into successful and unsuccessful trials. Significantly more leg slipping in the middle legs was present in failing trials than in successful trials (*P<0.05). (E) Gait pattern in an aged individual that successfully climbed the incline. Although the front legs often took multiple steps during inclined walking, these animals used a metachronal gait to surmount the incline. (F) Gait pattern in an aged individual that did not successfully climb the incline. Leg movements were not coordinated in this trial and leg slipping was extensive.

View larger version (19K): [in a new window]   Fig. 9. Responses evoked by tactile stimulation of the lateral edge of the abdomen in 61-week-old tethered cockroaches under intact and decapitated conditions and young intact animals. Young intact animals readily show escape behavior when stimulated (data from Schaefer and Ritzmann, 2001). By contrast, intact 61-week-old cockroaches did not escape when stimulated. Following decapitation, however, forward directional escape responses (Schaefer et al., 1994) were easily elicited in aged individuals.