In the study of the molecular origins of behaviour, the nematode Caenorhabditis elegans has emerged as a powerful tractable model. To fully understand an organism's behaviour, one must also understand the physics of the surrounding medium. The work discussed in this thesis focuses on the interactions of C. elegans with the environment it is most frequently observed in---a viscoelastic agar substrate covered by a thin liquid film.;We have used a micropipette-based force measurement technique to directly measure for the first time the forces involved in crawling locomotion of a microorganism. We found that substrate viscoelasticity introduces nonlinearities in the force-velocity relationships, and suggested physical mechanisms for the origin of these nonlinearities. Furthermore, we applied a modification of resistive force theory to recordings of nematode body shape and were able to obtain good agreement with simultaneously recorded lateral forces exerted by a crawling subject.
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