Our goal is to describe a specific case of a general process gaining traction amongst biologists: testing biological hypotheses with biomimetic structures that operate in bioinspired robots. As an example, we present MARMT (mobile autonomous robot for mechanical testing), a surface-swimmer that undulates a submerged biomimetic tail to power cruising and accelerations. Our goal was to test the hypothesis that stiffness of the body controls swimming behavior and that both stiffness and behavior can be altered by changes in the morphology of the vertebral column. To test this hypothesis, we built biomimetic vertebral columns (BVC) outfitted with variable numbers of rigid ring centra; as the number of centra increased the axial length of the intervertebral joints decreased. Each kind of BVC was tested in dynamic bending to measure the structure's apparent stiffness as the storage and loss moduli. In addition, each kind of BVC was used as the axial skeleton in a tail that propelled MARMT. We varied MARMT's tail-beat frequency, lateral amplitude of the tail, and swimming behavior. MARMT's locomotor performance was measured using an on-board accelerometer and external video. As the number of vertebrae in the BVC of fixed length increased, so, too, did the BVC's storage modulus, the BVC's loss modulus, MARMT's mean speed during cruising, and MARMT's peak acceleration during a startle response. These results support the hypothesis that stiffness of the body controls swimming behavior and that both stiffness and behavior can be altered by changes in the morphology of the vertebral column.
Climbing over chasms larger than step size is vital to fruit flies, since foraging and mating are achieved while walking. Flies avoid futile climbing attempts by processing parallax-motion vision to estimate gap width. To identify neuronal substrates of climbing control, we screened a large collection of fly lines with temporarily inactivated neuronal populations in a novel high-throughput assay described here. The observed climbing phenotypes were classified; lines in each group are reported. Selected lines were further analysed by high-resolution video cinematography. One striking class of flies attempts to climb chasms of unsurmountable width; expression analysis guided us to C2 optic-lobe interneurons. Inactivation of C2 or the closely related C3 neurons with highly specific intersectional driver lines consistently reproduced hyperactive climbing whereas strong or weak artificial depolarization of C2/C3 neurons strongly or mildly decreased climbing frequency. Contrast-manipulation experiments support our conclusion that C2/C3 neurons are part of the distance-evaluation system.Terrestrial locomotion is an important mode of translocation for Drosophila melanogaster flies as searching for food in the near-field and courtship occur during walking. A striking aspect of walking behaviour is the crossing of gaps of up to 1.7-times the fly's body length of 2.6 mm. Unmistakable leg-over-head strokes of the forelegs indicate an attempt. Therefore, turning back without trying to climb can reliably be distinguished from a futile attempt 1 . The probability for a climbing attempt is gap-width dependent. Gaps smaller than body size are usually overcome just by a large stride. Climbing behaviour becomes frequent at broader, yet surmountable gaps; then the initiation rate decreases with increasing width. Flies estimate gap width during approach by the parallax motion they perceive from the other side of the gap, not by binocular depth perception 1 . Distance estimation to objects in Drosophila relies on small-field motion vision 2 ; parallax motion but not looming of retinal images is used while walking. These picture shifts between foreground and background contain distance information because the distal side's image of a narrow gap moves faster over the retina than that of a wide gap. Gap-crossing behaviour is not just remarkable for this visually guided decision process, but also for its precise execution, requiring integration of mechanosensory, proprioceptive and visual information 1,3 . Forelegs stretch out far to reach the distal side of the gap, middle legs lift up the body to a vertical position within the gap, and hind legs are positioned as close as possible to the gap's proximal edge. No legged robot is as dexterous as these climbing flies. Understanding the neuronal control of gap crossing is therefore of applied interest in addition to providing insights into general biological processes such as decision making. However, the neuronal circuitry underlying climbing behaviour is largely unknown.Here, we report de...
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