When rain begins, flies often disappear, prompting questions about their ability to navigate wet conditions. For these small creatures, rainfall presents a significant challenge, transforming the open air into a hazardous environment. Understanding how flies interact with rain involves examining the physical forces at play and their behavioral responses to such conditions.
The Physics of Rain and Small Insects
Raindrops pose a substantial physical threat to flies due to the immense difference in scale. A typical raindrop, 0.5 to 6 millimeters in diameter, can be up to 50 times heavier than a fly. The impact of a single raindrop on a fly is comparable to a person being struck by a car. These raindrops descend at speeds of approximately 7 to 10 meters per second, or about 16 to 22 miles per hour.
The force of impact from such a fast-moving, massive object makes sustained flight impractical for most flies during precipitation. Water’s surface tension also becomes a significant factor for small insects. For tiny creatures, water acts like a sticky, viscous substance, potentially trapping them if their bodies become thoroughly wet. This combination of impact force and water’s adhesive properties creates an environment largely inhospitable for active flight.
Observing Fly Behavior in Rain
When rain starts, flies typically cease active flight and prioritize seeking shelter. Common hiding spots include the undersides of leaves, protected areas under eaves, or within crevices in tree bark and other structures. They might also retreat into dense vegetation or even enter human-made structures like buildings if access is available.
Wet wings become heavy and can stick together, severely impairing a fly’s ability to remain airborne. Smaller flies are particularly susceptible to the physical impact of raindrops, which can knock them out of the sky or cause injury. Additionally, the drop in temperature often associated with rainfall can slow down a fly’s metabolism, further reducing its capacity for sustained activity.
Coping with the Elements
Flies possess several adaptations that help them survive rainy conditions. Their remarkably low mass means that if hit by a raindrop, they tend to be pushed along with the drop rather than being crushed by its force. This allows them to “ride” the droplet for a brief moment, minimizing the impact force before detaching and regaining control. Their exoskeletons are also robust and can withstand buffeting without significant damage.
Flies have specialized water-repellent surfaces on their bodies, including tiny hairs or setae, which cause water to bead up and roll off. This property prevents them from becoming waterlogged or trapped by surface tension. If a fly is momentarily stunned or knocked down by a raindrop, it can often recover quickly, regaining balance to either resume flight or find shelter. Once the rain subsides and humidity levels rise, flies often become more active, venturing out to forage for food, find mates, and locate suitable breeding sites.