Springtails (Collembola) have been traditionally portrayed as explosive jumpers with incipient directional takeoff and uncontrolled landing. However, for these collembolans that live near the water, such skills are crucial for evading a host of voracious aquatic and terrestrial predators. We discover that semiaquatic springtails, Isotomurus retardatus , can perform directional jumps, rapid aerial righting, and near-perfect landing on the water surface. They achieve these locomotive controls by adjusting their body attitude and impulse during takeoff, deforming their body in midair, and exploiting the hydrophilicity of their ventral tube, known as the collophore. Experiments and mathematical modeling indicate that directional-impulse control during takeoff is driven by the collophore’s adhesion force, the body angle, and the stroke duration produced by their jumping organ, the furcula. In midair, springtails curve their bodies to form a U-shape pose, which leverages aerodynamic forces to right themselves in less than ~20 ms, the fastest ever measured in animals. A stable equilibrium is facilitated by the water adhered to the collophore. Aerial righting was confirmed by placing springtails in a vertical wind tunnel and through physical models. Due to these aerial responses, springtails land on their ventral side ~85% of the time while anchoring via the collophore on the water surface to avoid bouncing. We validated the springtail biophysical principles in a bioinspired jumping robot that reduces in-flight rotation and lands upright ~75% of the time. Thus, contrary to common belief, these wingless hexapods can jump, skydive, and land with outstanding control that can be fundamental for survival.

10Whether or not the micro swimmer Caenorhabditis elegans senses and respond to gravity is 11 unknown. We find that C. elegans aligns its swimming direction with that of the gravity vector 12 (positive gravitaxis). When placed in an aqueous solution that is denser than the animals, they 13 still orient downwards, indicating that non-uniform mass distribution and/or hydrodynamic 14 effects are not responsible for animal's downward orientation. Paralyzed worms and worms 15 with globally disrupted sensory cilia do not change orientation as they settle in solution, 16indicating that gravitaxis is an active behavior that requires gravisensation. Other types of 17 sensory driven orientation behaviors cannot explain our observed downward orientation. Like 18 other neural behaviors, the ability to respond to gravity declines with age. Our study establishes 19 gravitaxis in the micro swimmer C. elegans and suggests that C. elegans can be used as a 20genetically tractable system to study molecular and neural mechanisms of gravity sensing and 21 orientation. 22 23 24 2 Significance Statement 25Understanding how animals respond to gravity is not only of fundamental scientific interest, 26 but has clinical relevance, given the prevalence of postural instability in aged individuals. 27Determining whether C. elegans responds to gravity is important for mechanistic studies of 28 gravity sensing in an experimentally tractable animal, for a better understanding of nematode 29 ecology and evolution, and for studying biological effects of microgravity. Our experiments, 30which indicate that C. elegans senses and responds to gravity, set the stage for mechanistic 31 studies on molecular mechanisms of gravity sensing. 32When ≥ 1 and ≥ 3, over 73% and 95% of the animals are oriented, respectively, at a polar 131 angle > 90 o . We compute the concentration parameter  for our data by fitting the cumulative 132 distribution function (cdf) associated with equation 2 (SI-section S4) to the experimental one. 133When the animal is at depth d = 4 mm beneath the surface ~ 0.2 (nearly uniform distribution). 134As the animal's depth increases (the animal has more time to align with the direction of gravity), 135 the skewness of the KDE and the magnitude of increase as well For the well-fed WT animals 136  increases at the approximate rate of 0.07 per mm of depth until it asymptotes to ~ 4.3 at ~60 137 mm, and approximately retains this value at depths exceeding 60 mm. KDEs at depths 120 mm 138 < d < 200 mm nearly overlap (SI -Section S5). The inset in Fig. 3 depicts the concentration 139 parameter  as a function of the animal's depth (d, mm) beneath the liquid surface. The data is funded by the Ministry of Education of Taiwan, Global Networking Talent 3.0 Plan (GNT3.0),

Nematodes such as Caenorhabditis elegans are heavier than water. When submerged in water, they settle to the bottom surface. Observations reveal that the animals do not lie flat on the bottom surface, but remain substantially suspended above the surface through continuous collisions with the surface, while maintaining their swimming gaits. Consequently, the swimming animals follow the bottom surface topography. When the bottom surface is inclined, the animals swim up or down along the incline. As the magnitude of the gravitational force can be easily estimated, this behaviour provides a convenient means to estimate the animal's propulsive thrust. The animals' tendency to follow the surface topography provides a means to control the swimmers' trajectories and direction of motion, which we demonstrate with a saw tooth-like ratchet that biases the animals to swim in a selected direction. The animals can also serve as surface topography probes since their residence time as a function of position provides information on surface features. Finally, we take advantage of surface following to construct a simple motility-based sorter that can sort animals based on genotype and state of health. IntroductionMotility assays for nematodes, such as Caenorhabditis elegans, often monitor, from above, the motion of animals suspended in aqueous solutions. In most cases, the animals are observed to swim. Caenorhabditis elegans is, however, heavier than water [1] and sediments to the bottom. Although nematodes' sedimentation per se has not been investigated extensively, nematologists have known for a long time that nematodes sediment in a gravitational field and have taken advantage of this phenomenon to isolate animals (i.e. in the Baermann funnel method) [2]. Nematode settling is also used extensively in various assay preparations [3].That gravitational forces play a significant role in nematodes' hydrodynamics is hardly surprising. To demonstrate that gravitational forces impact nematodes' swimming trajectories, we carry out a simple scaling analysis. Fluid mechanicians define the gravity parameter G ¼ ðr a À r l Þ=r l  ga 2 =nU, representing the ratio of the gravitational body force ðr a À r l Þga 2 L and the viscous force mUaL=a. In the above, r a and r l are, respectively, the density of the animal and the suspending liquid, L is the length of the animal, g is gravitational acceleration, a is the animal's radius, m is the suspending liquid's viscosity, v ¼ m/r l , and U is the animal's velocity. When an adult C. elegans is suspended in water, ðr a À r l Þ=r l 0:07 [1]. Adult C. elegans has a radius a 40 mm and length L 1 mm. The liquid kinematic viscosity n 10 26 m 2 s 21 and the adult animal's velocity U 200 mm s 21. G is of order 1, indicating that gravitational forces are as important as propulsive forces and significantly impact the animal's swimming trajectory.What happens to the animal once it settles to the bottom? One might naively assume that the animal lies flat on the bottom surface. If this were the case, the anima...

In this paper, a new continuum traffic flow model is proposed, with a lane-changing source term in the continuity equation and a lane-changing viscosity term in the acceleration equation. Based on previous literature, the source term addresses the impact of speed difference and density difference between adjacent lanes, which provides better precision for free lane-changing simulation; the viscosity term turns lane-changing behavior to a "force" that may influence speed distribution. Using a flux-splitting scheme for the model discretization, two cases are investigated numerically. The case under a homogeneous initial condition shows that the numerical results by our model agree well with the analytical ones; the case with a small initial disturbance shows that our model can simulate the evolution of perturbation, including propagation, dissipation, cluster effect and stop-and-go phenomenon.