Meeting Abstract

89.1  Thursday, Jan. 7  Ontogenetic scaling of overall body properties in Manduca caterpillars and its implications on the use of a hydrostatic skeleton LIN, H.*; PAETSCH, C.R.; SLATE, D.J.; DORFMANN, A.L.; TRIMMER, B.A.; Tufts University; Tufts University; Tufts University; Tufts University; Tufts University huai-ti.lin@tufts.edu

Unlike most other soft-bodied animals, caterpillars such as Manduca sexta have an internal respiratory system consisting of air-filled tubes called trachea. Because air can be compressed or even expelled during locomotion they do not have a constant volume, incompressible coelom as found in annelids. This invalidates one of the fundamental assumptions used to model closed-vessel hydrostatic skeletons. Instead, we have found that caterpillars use the substrate as their external skeleton during normal locomotion with a relatively small amount of stiffening from turgor pressure. A more conventional hydrostatic skeleton (and its associated metabolic inefficiency) may be reserved for casting or other ballistic behaviors requiring a higher pressure. In this study, we modeled the caterpillar body as a thin-wall inflatable cylinder with fiber reinforcement. Under normal condition, the longitudinal load is taken by the muscles while the circumferential stress is held by the cuticle. Using known mechanical properties of Manduca muscles and cuticle, a simplified finite element model was constructed where flexural stiffness was computed numerically. By changing the cylinder dimensions and incorporating air cavities, we could calculate the work associated with establishing given pressures and stiffness. To provide input for the model, we directly measured the body density and body pressure of Manduca across different developmental stages and sizes. By comparing the model predictions to caterpillar behaviors we can determine some of the biomechanical and developmental constraints in a low pressure, climbing, soft bodied, terrestrial animal.