Caterpillar locomotion

Movements in soft-bodied animals are difficult to understand due to the ambiguous substrate interaction and the continuous morphing body. For decades, biologists quantified soft-bodied animal movements using kinematics (external observation) and geometric models. When I started my Ph.D research, I realize that the best way to resolve some ambiguity in soft-bodied motion is to measure the force transmission. After all, the external movement can be deceiving when the subject is highly deform-able. Caterpillar are the juvenile form of butterflies and moths (lepidoptera) and represents one of the most diverse animal groups on earth. Except the head capsule which houses the mouth parts and the six true legs that help with eating, they accomplish most of their behaviors without any form of articulation (no internal skeleton or exoskeleton). Among the soft-bodied animals, the caterpillar is unique in the use of discrete substrate attachments which gives us an great opportunity to study force distribution during locomotion. So I set out to design a mini force plate array to measure the substrate reaction forces from a crawling caterpillar.

CaterpillarForceBeam

I learned a lot about instrumentation in the process and also developed a novel strain-gauge arrangement for bi-axial force sensing. The substrate reaction force data also helped us understand how the caterpillar preload its body in tension to maintain body stability during a leg shift.

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