Aerodynamics and the role of the earth's electric field in the spiders' ballooning flight.

Some spiders aerially disperse relying on their fine fibres. This behaviour has been known as 'ballooning'. Observations on the ballooning behaviour of spiders have a long history and have more recently received special attention, yet its underlying physics is still poorly understood. It was traditionally believed that spiders rely on the airflows by atmospheric thermal convection to do ballooning. However, a recent experiment showed that exposure to an electric field alone can induce spiders' pre-ballooning behaviours (tiptoe and dropping/dangling) and even pulls them upwards in the air. The controversy between explanations of ballooning by aerodynamic flow or the earth's electric field has long existed. The major obstacle in studying the physics of ballooning is the fact that airflow and electric field are both invisible and our naked eyes can hardly recognise the ballooning silk fibres of spiders. This review explores the theory and evidence for the physical mechanisms of spiders' ballooning connects them to the behavioural physiology of spiders for ballooning. Knowledge gaps that need to be addressed in future studies are identified.

Wing Design in Flies: Properties and Aerodynamic Function.

The shape and function of insect wings tremendously vary between insect species. This review is engaged in how wing design determines the aerodynamic mechanisms with which wings produce an air momentum for body weight support and flight control. We work out the tradeoffs associated with aerodynamic key parameters such as vortex development and lift production, and link the various components of wing structure to flight power requirements and propulsion efficiency. A comparison between rectangular, ideal-shaped and natural-shaped wings shows the benefits and detriments of various wing shapes for gliding and flapping flight. The review expands on the function of three-dimensional wing structure, on the specific role of wing corrugation for vortex trapping and lift enhancement, and on the aerodynamic significance of wing flexibility for flight and body posture control. The presented comparison is mainly concerned with wings of flies because these animals serve as model systems for both sensorimotor integration and aerial propulsion in several areas of biology and engineering.

The Importance of a Filament-like Structure in Aerial Dispersal and the Rarefaction Effect of Air Molecules on a Nanoscale Fiber: Detailed Physics in Spiders' Ballooning.

Many flying insects utilize a membranous structure for flight, which is known as a "wing." However, some spiders use silk fibers for their aerial dispersal. It is well known that spiders can disperse over hundreds of kilometers and rise several kilometers above the ground in this way. However, little is known about the ballooning mechanisms of spiders, owing to the lack of quantitative data. Recently, Cho et al. discovered previously unknown information on the types and physical properties of spiders' ballooning silks. According to the data, a crab spider weighing 20 mg spins 50-60 ballooning silks simultaneously, which are about 200 nm thick and 3.22 m long for their flight. Based on these physical dimensions of ballooning silks, the significance of these filament-like structures is explained by a theoretical analysis reviewing the fluid-dynamics of an anisotropic particle (like a filament or a high-slender body). (1) The filament-like structure is materially efficient geometry to produce (or harvest, in the case of passive flight) fluid-dynamic force in a low Reynolds number flow regime. (2) Multiple nanoscale fibers are the result of the physical characteristics of a thin fiber, the drag of which is proportional to its length but not to its diameter. Because of this nonlinear characteristic of a fiber, spinning multiple thin ballooning fibers is, for spiders, a better way to produce drag forces than spinning a single thick spider silk, because spiders can maximize their drag on the ballooning fibers using the same amount of silk dope. (3) The mean thickness of fibers, 200 nm, is constrained by the mechanical strength of the ballooning fibers and the rarefaction effect of air molecules on a nanoscale fiber, because the slip condition on a fiber could predominate if the thickness of the fiber becomes thinner than 100 nm.

An observational study of ballooning in large spiders: Nanoscale multifibers enable large spiders' soaring flight.

The physical mechanism of aerial dispersal of spiders, "ballooning behavior," is still unclear because of the lack of serious scientific observations and experiments. Therefore, as a first step in clarifying the phenomenon, we studied the ballooning behavior of relatively large spiders (heavier than 5 mg) in nature. Additional wind tunnel tests to identify ballooning silks were implemented in the laboratory. From our observation, it seems obvious that spiders actively evaluate the condition of the wind with their front leg (leg I) and wait for the preferable wind condition for their ballooning takeoff. In the wind tunnel tests, as-yet-unknown physical properties of ballooning fibers (length, thickness, and number of fibers) were identified. Large spiders, 16-20 mg Xysticus spp., spun 50-60 nanoscale fibers, with a diameter of 121-323 nm. The length of these threads was 3.22 ± 1.31 m (N = 22). These physical properties of ballooning fibers can explain the ballooning of large spiders with relatively light updrafts, 0.1-0.5 m s-1, which exist in a light breeze of 1.5-3.3 m s-1. Additionally, in line with previous research on turbulence in atmospheric boundary layers and from our wind measurements, it is hypothesized that spiders use the ascending air current for their aerial dispersal, the "ejection" regime, which is induced by hairpin vortices in the atmospheric boundary layer turbulence. This regime is highly correlated with lower wind speeds. This coincides well with the fact that spiders usually balloon when the wind speed is lower than 3 m s-1.