A Model of Larval Biomechanics Reveals Exploitable Passive Properties for Efficient Locomotion

To better understand the role of natural dynamics in motor control, we have constructed a mathematical model of crawling mechanics in larval

Drosophila

.

The model accounts for key anatomical features such as a segmentally patterned, viscoelastic outer body wall (cuticle); a non-segmented inner cavity (haemocoel) filled with incompressible fluid that enables visceral pistoning; and claw-like protrusions (denticle bands) giving rise to asymmetric friction.

Under conditions of light damping and low forward kinetic friction, and with a single cuticle segment initially compressed, the passive dynamics of this model produce wave-like motion resembling that of real larvae. The presence of a volume-conserving hydrostatic skeleton allows a wave reaching the anterior of the body to initiate a new wave at the posterior, thus recycling energy. Forcing our model with a sinusoidal input reveals conditions under which power transfer from control to body may be maximised. A minimal control scheme using segmentally localised positive feedback is able to exploit these conditions in order to maintain wave-like motion indefinitely. These principles could form the basis of a design for a novel, soft-bodied, crawling robot.