Effective behavior is the result of the properly timed and coordinated action of muscles. Therefore, a fundamental question in motor systems
neurobiology is how the central nervous system of any animal is able to generate such properly timed and coordinated action. This question is
often investigated by studying a repetitive behavior like walking because such repetitive (rhythmic) behavior is usually easier to evoke and
in principle easier to understand than non-rhythmic activity.
Insect walking has often been used as a model system for the study of rhythmic behavior, and research over the past several decades has
revealed much about the insect locomotor system. First, in common with other rhythmic behavior, the alternating action of antagonistic
muscle pairs (leg extensors and flexors) is generated by networks of neurons that can produce rhythmic output without needing any external
timing signals. Such networks are called central pattern generators (CPGs). There is at least one CPG per leg; some evidence suggests that
there may even be one per joint. The CPGs are coupled together via interneurons so that the activity of one can influence the activity of
another. Second, sensory feedback from the moving legs is in most cases critical for the proper coordination of the legs during walking.
In some insects (such as stick insects like Carausius morosus) the requirement for sensory feedback is so strong that coordination
virtually disappears if it is eliminated. In other insects (such as cockroaches like Periplaneta americana), the need for sensory
feedback may be reduced during fast walking.
The effects of sensory feedback can be seen in walking insects under certain conditions. Nearly all insects use a characteristic and
stereotypical gait (sequence of leg movements) when they walk. In this gait, called the alternating triangle gait, the front and rear
legs on one side of the body and the middle leg on the other side move more or less together and alternate in their movements with the
triangle of the remaining three legs. You can see the gait pattern in the video clip. Click on the icon labeled "Intact." You may be
able to step through the video frame by frame. The video was acquired at a rate of 200 frames per second.
If an insect loses a middle leg, the effect is dramatic. If the alternating triangle gait were maintained, the insect would have no
support on one side of the body when the front and rear legs on the side with the missing leg are lifted. Due to the absent sensory
input from the missing leg and the changed sensory input from the legs that remain, the insect immediately adapts to the loss of one
or more legs. You can see the effects of the loss of two middle legs. Click on the icon labeled "Legs missing." The video was acquired
at a rate of 200 frames per second.