Quadrupedal gaits in hexapod animals - inter-leg coordination in free-walking adult stick insects

The analysis of inter-leg coordination in insect walking is generally a study of six-legged locomotion. Since decades the stick insect Carausius morosus is instrumental for unraveling rules and mechanisms that control leg coordination in hexapeds. We analyzed inter-leg coordination in Carausius morosus that walked freely straight paths on plane surfaces with different slopes. Consecutive 1.7 second sections were assigned inter-leg coordination patterns (gaits) based on foot fall patterns. Regular gaits, i.e. wave, tetrapod or tripod gaits occurred depending on surface slopes. Most often tetrapod gaits were observed, wave gaits only occurred on 90° inclining slopes and tripod gaits most often on 15° declining slopes, i.e. in 40% of sections. Depending on slope 36% to 66% of sections were assigned irregular gaits. Irregular gaits were mostly due to multiple stepping in front legs, which is perhaps probing behavior, not phase coupled to middle legs’ cycles. In irregular gaits middle and hind leg coordination was regular, related to quadrupedal walk and wave gaits. Apparently front legs uncouple from and couple into the walking system without compromising middle and hind leg coordination. In front leg amputee remaining legs are strictly coordinated. In hind and middle leg amputees front legs continued multiple stepping. Middle leg amputees’ coordination was maladapted with front and hind legs performing multiple stepping or ipsilateral legs being in simultaneous swing. Thus, afferent information from middle legs might be necessary for a regular hind leg stepping pattern.