Mosquitoes survive raindrop collisions by virtue of their low mass

Abstract

In the study of insect flight, adaptations to complex flight conditions such as wind and rain are poorly understood. Mosquitoes thrive in areas of high humidity and rainfall, in which raindrops can weigh more than 50 times a mosquito. In this combined experimental and theoretical study, we here show that free-flying mosquitoes can survive the high-speed impact of falling raindrops. High-speed videography of those impacts reveals a mechanism for survival: A mosquito's strong exoskeleton and low mass renders it impervious to falling drops. The mosquito's low mass causes raindrops to lose little momentum upon impact and so impart correspondingly low forces to the mosquitoes. Our findings demonstrate that small fliers are robust to in-flight perturbations.surface tension | splash | acceleration O ne of the technological feats of the 21st century is the construction of insect-sized flying robots, micro-airborne vehicles (MAVs), made possible by rapidly shrinking manufacturing and elecronics (1-7). These robots have numerous applications such as deployment in swarms for surveillance and searchand-rescue operations. In parallel with the engineering of MAVs, vigorous efforts continue to be made into understanding flight in the natural world, such as by birds and insects (8-11). Much progress has been made in understanding straight-path flight in unidirectional flow. However, much remains to be understood about the abilities of birds and insects to fly through complex conditions such as wind and rain. Such knowledge clearly has implications for ecology in terms of understanding the evolution of animals in rain forests and near waterfalls. The adaptations of these animals may also serve engineering via biological inspiration for the design of robust MAVs.Previous studies on bats have shown that rain doubles their energetic cost of flight (12). The remainder of our knowledge of the effect of precipitation is restricted to large aircraft, although they operate upon very different principles from flapping fliers. Field testing on the effects of heavy rain on aircraft (13) confirms that precipitation generally hinders locomotion. Aircraft experience greater drag (2-5%), reduced lift (7-29%), and a reduction in stall angle of 1-5°, as measured (14, 15) during a rainfall intensity of 100-1;000 mm∕h. Aircraft can reduce these losses by using wing designs that can funnel rivulets and control their diameter. These design principles may explain some of the water-repellent features common in birds' wings (16). However, they clearly do not apply for much smaller fliers such as insects which are closer in size to raindrops.Flying insects likely perceive raindrop impacts as in-flight perturbations. There have been many studies of such perturbations, although none have considered the influence of a wetting fluid such as rain. For instance, bees exposed to turbulent air resist rolling instabilities by extending their legs to increase their moment of inertia (17). Following in-flight perturbation, fruit flies actively restabilize themselv...

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“…Perched hummingbirds shake their wings, head and body at accelerations of up to 30g, expelling nearly 90% of the water adhered to their plumage over a period of only several hundred milliseconds [39]. Insects, which are otherwise well known for their hydrophobic cuticles, also can become wet in rain, with a doubling of body mass under certain environmental conditions [40]. Even some vertebrates [39,41] as well as insects (e.g.…”

“…Species with exposed surface area equal to or less than drop size can experience only partial momentum transfer during impacts. However, such reduced transfer can result in substantial roll, pitch and yaw torques, sometimes even causing impact with the ground [40] (electronic supplementary material, Videos S1 and S2; figure 3a). By contrast, larger animals will experience much higher levels of momentum transfer during drop impacts (figure 3b), albeit distributed much more evenly across the body and wings.…”

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