Flies are highly active during the day, leading many to wonder about their activity at night. The question of whether flies can fly at night involves their biology, especially their visual systems and environmental interactions. While the answer is not a simple yes or no for all species, understanding scientific principles clarifies their behavior in low-light conditions.
Fly Vision and Nighttime Activity
The primary reason most common flies are less active at night lies in their unique visual system. Flies possess compound eyes, composed of thousands of individual light-sensing units called ommatidia. Each ommatidium acts like a tiny, independent lens, contributing to a mosaic-like image of the world. This visual arrangement grants flies a wide field of view and ability to detect motion, allowing them to react quickly to changes in their surroundings.
Flies are particularly sensitive to ultraviolet (UV) light, a part of the spectrum that humans cannot see. This sensitivity aids navigation and locating food sources, as many plants reflect UV light. While flies can adjust their eyes to dim conditions, their vision suffers in low light due to the reduced number of photons available. To compensate, they employ a neural strategy called summation, integrating light signals over longer periods or from neighboring ommatidia, which leads to a blurrier and less acute perception. Consequently, species like the common house fly, which are primarily diurnal, become inactive at night and seek resting spots.
Diurnal and Nocturnal Fly Species
While many common flies are day-active, activity patterns vary widely among species. The concept of diurnal (day-active) and nocturnal (night-active) species applies to flies. House flies and many fruit flies are diurnal insects, relying heavily on bright light for their activities.
However, certain fly species are exceptions to this general rule, active after sunset. Mosquitoes, for instance, are nocturnal, with females often seeking blood meals. Some midges and dobsonflies are also nocturnal. These night-active species possess adaptations to enhance their vision in dim light, such as larger ommatidia or specialized eye structures like superposition eyes. These adaptations allow them to gather more light, improving their sensitivity, though sometimes at the cost of image resolution.
Environmental Factors Affecting Fly Activity
Beyond vision and species adaptations, environmental factors influence fly activity. Temperature plays a significant role, as flies are cold-blooded creatures whose metabolism and movement depend on ambient temperature. They are active in warm conditions but become sluggish or immobile when temperatures drop. Optimal temperatures for activity and reproduction are often above 75 degrees Fahrenheit.
Humidity is another factor, with many flies thriving in warm, moist environments. High humidity can accelerate their reproductive cycles and contribute to sustained activity. Fruit flies, for example, show a preference for around 70% relative humidity, and flies possess specialized neurons to detect humidity levels, which guides their behavior.
Artificial light sources can impact fly activity at night. Many fly species exhibit positive phototaxis, drawing them to light. This attraction can disorient them, disrupting their natural navigation cues and leading to behaviors like circling or crashing. Such light pollution can have negative effects, causing exhaustion, increased predation, and even fatality for insects.