How do flying insects like important pollinators locate odor sources in the great outdoors, despite encountering highly variable wind conditions? They use odor plumes – which travel like smoke and form when the wind blows odor molecules from their source – to track down sources such as flowers or pheromones.
But wind tunnels are typically unable to replicate realistic outdoor wind conditions. A study published in the AIP journal Physics of Fluids explores microscale wind conditions in various outdoor environments to investigate what flying insects might experience while tracking odor plumes.
Study co-authors Jaleesa Houle and Floris Van Breugel from the University of Nevada at Reno assessed the mechanical turbulence produced by ambient wind flowing over surface roughness elements such as buildings, trees, and fences and its role in odor plume tracking.
“Since we’re studying wind dynamics within the surface roughness sublayer, most known atmospheric similarity theories that describe properties of the wind profile are not applicable,” said Houle. “So, we use statistical analysis to find both spatial and temporally significant correlations between wind measurements for various sites where we collected data.”
The researchers collected near-surface wind data from several sage steppe (shrub-filled grassland), forest, and urban areas in Northern Nevada and discovered near-surface wind direction is often highly variable over timescales of less than 10 minutes. They also found wind direction variability to be consistently higher in environments with greater surface complexity (urban areas) and lower at higher wind speeds.
“This is important because insects are typically tracking odor plumes in lower wind speeds, which indicates they are somehow making sense of the high directional variability they encounter,” said Houle. “Turbulence intensity is strongly correlated with standard deviations in wind direction, which might be useful for future wind tunnel experimental designs aimed at recreating more ‘natural’ winds.”
Based on their findings, Houle and van Breugel hypothesize an optimal range of wind speed and environmental surface complexity may exist to help insects locate an odor source.
“Further experiments will be needed to test our hypothesis and may help us better understand the implications of land fragmentation on the success of ecologically significant plume tracking insects, such as pollinators,” said Houle.
“Beyond this, our results give a compelling reason for researchers to focus on increasing directional variability in wind tunnel studies if they want to uncover plume tracking behaviors that more closely resemble what we might see in nature.”
Next, the researchers will apply their findings to plume tracking wind tunnel experiments and a series of outdoor studies.
“During the summer, we plan to test our hypothesis regarding the types of wind conditions insects might prefer while tracking odor plumes,” said Houle. “In the lab, we’re actively looking for ways to create greater directional variability to better mimic natural wind.”
Insect flight is a remarkable natural phenomenon that has intrigued scientists for centuries. Despite their seemingly erratic patterns, insects are actually incredibly skilled fliers, capable of executing complex maneuvers that even our most advanced aircraft can’t duplicate.
At the heart of an insect’s flying ability are its wings, which are made of a thin, flexible cuticle – the same material that makes up the insect’s exoskeleton. The wings are incredibly light, yet strong enough to withstand the rapid, vigorous beating required for flight.
Insects typically have two pairs of wings, although in some species, like flies (Diptera), the second pair of wings has evolved into structures known as halteres that help in balance and navigation.
Insect flight involves two primary wing-beating patterns: the figure-eight and the clap-and-fling.
This is the most common pattern and is employed by bees, flies, and many other insects. The insect moves its wings in a figure-eight pattern, creating a swirl of air (vortex) above the wing that produces lift. It’s an efficient way of flying that allows insects to hover in place, fly backward, and quickly change direction.
In this method, used by smaller insects like thrips and some kinds of beetles, the insect claps its wings together above its body and then flings them apart. When the wings fling apart, the air rushes in to fill the void, creating a vortex that generates lift.
One of the major differences between insect flight and bird or plane flight is how lift is created. Birds and airplanes create lift by moving forward, with air flowing faster over the top of the wing than the bottom due to its shape (an effect described by Bernoulli’s principle). Insects, on the other hand, generate lift by flapping their wings back and forth at high frequencies.
Insects are also able to control the angle at which their wings beat, known as the “angle of attack,” allowing them to change direction quickly or hover in place. They have an incredible amount of control over their flight, capable of near-instant changes in speed and direction.
The ability of insects to fly has played a critical role in their success as a group, aiding in food acquisition, escape from predators, and dispersion to new habitats. Despite their small size, insects have evolved one of the most effective forms of flight in the animal kingdom.