Biomechanical Strategies for Locomotion in Soft-bodied Animals.
Abstract: Soft-bodied animals have relatively few mechanical constraints on
movements. This freedom is expected to impose a great challenge to muscle force
transmission and body coordination. I have used the caterpillar as a model system to
explore the role of soft mechanics in the control of locomotion. Caterpillars are extremely
successful herbivores that roam on complex branched structures.... read moreThey have multiple discrete
soft appendages (prolegs) that attach their bodies to the substrate and can be released on
demand. This well-defined substrate interaction makes caterpillars ideal for studying force
transmission in soft-bodied animals. In this study, a custom two dimensional force sensor
array measures ground reaction forces from the caterpillar prolegs during crawling. The
data show persistent inter-segmental tensions propagating forward along the caterpillar's
body. By loading itself against the substrate, the caterpillar constrains its mechanics to
preferentially stretch and achieve locomotion (a strategy I call "environmental skeleton").
While the substrate provides essential support for crawling caterpillars, inching
caterpillars have to rely mostly on their hydrostatic skeletons. A field survey of
caterpillar gait diversity reveals many different proleg configurations and their
associated motor sequences. In caterpillars with partial proleg reduction, we found various
intermediate gaits with characteristics of crawling and inching. The transition from
crawling to inching seems to require two major evolutionary changes. The reduction of
mid-body prolegs allows the body to loop away from the substrate, and strengthening the
hydrostatic skeleton prompts the body to flex instead of compress. A model based on tissue
properties of Manduca caterpillar suggests that smaller
hydrostats are more stable, consistent with the observation that inching caterpillars tend
to be smaller. This gait transition was simulated in several soft-bodied robots. From a
simple crawling gait, pacing the motor pattern and removing mid body attachment produce an
inching gait comparable to inchworm locomotion. Further, my soft-bodied robots demonstrate
how nonlinear loading and large deformation result in behaviors that are not sensitive to
the variations in motor commands. Clever morphological designs therefore allow us to embed
simple adaptive control in the soft structures (e.g. pre-stressing the body for a dynamic
event). We expect to find similar control strategies in soft-bodied
Thesis (Ph.D.)--Tufts University, 2011.
Submitted to the Dept. of Biology.
Advisor: Barry Trimmer.
Committee: Barry Trimmer, Luis Dorfmann, Gary Leisk, Harry Bernheim, Sara Lewis, Jason Rife, and Sharon Swartz.
Keywords: Biomechanics, Robotics, and Entomology.read less