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.