A new behavioral experiment --- walk the substrate

From my caterpillar GRF data, I discovered that in almost any instance the substrate is stretching the animal axially. I was intrigued by the idea that such a soft-bodied animal might be actually "depending" on the substrate stiffness to achieve neccessary deformation during locomotion. So, I thought maybe the stiffness of the substrate would affect the crawling performance in a very predictive way. The first test that came to my mind was to suspend the animal from the head capsule and the rear horn (see photos). By feeding the animal different substrates, they could "walk the substrate backward" by doing their normal forward locomotion in my suspension setup. Interestingly, they crawl just fine with a balsa wood. As I reduced the stiffness of the substrate by changing from wood to rubber and eventually to a soft wire, the caterpillar started to buckle the substrate and failed to produce the stereotypic locomotion pattern. This is the first behaviroal support for the "environmental skeleton hypothesis".

The roles of thoracic legs in caterpillar locomotion

We all know that the six thoracic legs are the true legs that will be retained through metamorphosis in lepidoptera. However, amputation of these limbs does not seem to prevent a Manduca caterpillar from crawling around. Nevertheless, caterpillars appear to use them constantly during each crawl cycle. Originally we considered them as probing devices for sensing the substrate ahead, until I analized the GRF data from thoracic legs. Besides that fact that each leg pair could take up as much body weight as any pair of prolegs, they also exert quite significant amount of forward pull during each crawl. What happened to the amputated caterpillars I cannot figure, but thoracic legs definitely have a important role in normal locomotor performance.