Early papers describe caterpillar crawling as legged peristalsis, but recent work suggests that caterpillars use a tension-based mechanism that helps them to exploit arboreal niches. Caterpillars are not obligate hydrostats but instead use their strong grip to the substrate to transmit forces, in effect using their environment as a skeleton. In addition, the gut which accounts for a substantial part of the caterpillar's weight, moves independently of the body wall during locomotion and may contribute to crawling dynamics. Work-loop analysis of caterpillar muscles shows that they are likely to act both as actuators and energy dissipaters during crawling. Because caterpillar tissues are pseudo-elastic, and locomotion involves large body deformations, moving is energetically inefficient. Possession of a soft body benefits caterpillars by allowing them to grow quickly and to access remote food sources safely
Animals that must transition from horizontal to inclined or vertical surfaces typically change their locomotion strategy to compensate for the relative shift in gravitational forces. The species that have been studied have stiff articulated skeletons that allow them to redistribute ground reaction forces (GRFs) to control traction. Most also change their stepping patterns to maintain stability as they climb. In contrast, caterpillars, most of which are highly scansorial, soft-bodied, and lack rigid support or joints, can move with the same general kinematics in all orientations. In this study, we measure the GRFs exerted by the abdominal prolegs of Manduca sexta (Linnaeus) during locomotion. We show that, despite the orthogonal shift in gravitational forces, caterpillars use the same tension-based environmental skeleton strategy to crawl horizontally and to climb vertically. Furthermore, the transition from horizontal to vertical surfaces does not seem to require a change in gait; instead gravitational loading is used to help maintain a stance-phase body tension against which the muscles can pull the body upwards.
crawling is dominated by the abdominal movements and continues even if the thoracic legs are removed.
Debrained animal (46 h postop) transitioning from horizontal (not shown) to vertical orientation. Motion occurs in all three directions (forward, up, and to the right). Each frame represents +6 s.
thoracic legs are important as anchors that allow the rest of the body to develop and release tension. As the wave of released tension reaches the thoracic legs, they then move forward to re-establish the animal’s resting length (Lin and Trimmer 2010a). Manduca are still able to crawl horizontally after the thoracic legs have been removed or bonded to one another (Lin and Trimmer 2010a); however, it is possible that body weight presses the thorax against the substrate to generate enough friction to maintain normal crawling.
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Climbing by a second-day fifth instar Manduca with its thoracic legs removed. Successive frames show the stepping pattern in a single crawl cycle. Dots (red) indicate a proleg in stance and white arrows show prolegs in swing phase. Manduca is able to climb the rigid substrate using only the prolegs, but this gait does not resemble normal crawling. Each step is very short and without an anterior anchor point the anterior abdominal segments are compressed together.
Recent work on M. sexta locomotion demonstrates that during horizontal crawling, the caterpillar keeps its body in tension and transmits forces through the substrate to the rest of its body. This has been described as an environmental skeleton (Lin and Trimmer 2010a,b). The present study examined a larger dataset of horizontal crawling and vertical crawling. The data show that while each pair of prolegs has a distinct average substrate reaction force profile, there is a large variability in both the magnitude and time course of forces within each body segment (Figs. 1 and 2). This is in contrast to rigid bodied animals of similar or larger weights, where GRFs are highly reproducible and largely speed dependent