Mechanical design of the legs of Dolomedes aquaticus - Novel approaches to quantify the hydraulic contribution to joint movement and to create a segmented 3D spider model
Recent findings of integrative studies of locomotion have revealed that the principles of exploiting natural interactions of the moving body and its mechanical coupling with its environment are essential for efficient locomotion. Thus agility depends as much on mechanical design as on neural control. This work investigates specific aspects of mechanical design of spiders relevant to agile locomotion. Like other arthropods, spiders are capable of agile movement, but the anatomical mechanisms underlying this behavior are somewhat unique: instead of leg articulation being solely driven by muscle movement, spiders employ a hydraulic system to extend certain joints exclusively by hemolymph pressure. Thus the mechanical design of their legs is different from that of insects examined in previous studies. This makes spiders particularly interesting for further research in this area. The methods developed in this study provide a quantitative description of the angle-volume characteristics for femur-patella and tibia-metatarsus joints in the nursery web spider Dolomedes aquaticus, a semi aquatic arachnid from New Zealand. I designed an apparatus based on the principle of joint volume shift caused by passive joint movement to measure the displacement of hemolymph as a leg is flexed. Computational image analysis of video recordings from experimental trials was achieved using a custom-made algorithm. I fitted exponential, polynomial, power-law and hyperbolic models to the angle-volume data and used Akaike Information Criterion (AIC) to evaluate how well each model described the hydraulic characteristic of each joint. Based on AIC values the polynomial model had the best fit. Angle-volume characteristics for tibia-metatarsus joints revealed a continuous increase in volume-shift with more posterior position of the individual leg. For femur-patella joints the angle-volume characteristic of the most posterior leg was also the most pronounced. In contrast to the tibia-metatarsus joint the angle-volume characteristic of the anterior third leg showed the least volume-shift while the anterior second leg was almost equal to that of the most posterior leg. Results for standardized angle-volume characteristics suggest that joint volume in D. aquaticus scales according to geometric similarity. However, no consistent pattern for the standardization factor could be identified. In addition to the angle-volume characteristics a 3D model of D. aquaticus was reconstructed. I used modern micro-CT technology to generate a surface rendering dataset of a specimen. This technique provided sufficient accuracy to reproduce the topography of the external geometry. The surface model was segmented using position of joint axes as a reference. Segments were reconstructed manually using simple mesh geometries which were superimposed on the CT surface model. The resulting 3D model closely resembled the segmented exoskeleton and provides a platform for the construction of a fully functional biomechanical model of this species which will allow numerical simulations of inverse and forward-dynamics. Mechanical leg design is expected to be a key determinant of gait and locomotor efficiency. This study represents a further step towards an integrative analysis of mechanical leg design and its effect on locomotor dynamics. The findings contribute to an integrative understanding of the unique locomotor strategy employed by spiders. Furthermore, information and data are provided for computational modelling and technical applications in the field of robotics.
Advisor: Paulin, Michael G.
Degree Name: Master of Science
Degree Discipline: Zoology
Publisher: University of Otago
Keywords: arthropod; arachnid; Dolomedes aquaticus; legged locomotion; mechanical design; hydraulic leg joint; 3D model; robotics
Research Type: Thesis