House flies are a common pest that can be found in homes worldwide. A characteristic behavior of house flies is that they seem to be attracted to light sources. Upon entering a room, house flies may immediately fly towards lamps, windows, or other illuminated areas. This raises the question – do house flies actually get attracted to light? Or is their navigation towards brightness coincidental?
Understanding house fly light attraction is important for controlling these pests. If light does lure flies, it could potentially be used strategically in traps and other management approaches. On the other hand, if illumination is irrelevant to flies, it would be useless to target light sources in fly control efforts.
In this article, we’ll explore the evidence behind house fly attraction to light. Key points include:
House flies have eyes sensitive to light
House flies have specialized eyes that allow them to detect light across a wide spectrum. Their eyes contain thousands of individual units called ommatidia. Each ommatidium functions like a tiny lens, focusing light towards photoreceptor cells. Signals from these photoreceptors are sent to the fly’s brain, enabling them to see their surroundings.
The ommatidia are distributed across the fly’s head to give a broad field of vision. House flies can see light flickering up to 200-300 times per second. For comparison, humans see flicker up to 60 times per second. So flies have much faster vision capable of detecting the most subtle illumination changes.
This advanced eye structure indicates that house flies are highly visual creatures. Light is a primary way that they receive environmental information. The next question is how flies behaviorally react to the light they see.
Flies innately move towards light
Several studies have shown that house flies instinctively move towards light instead of away from it. This suggests they are positively phototactic.
In laboratory experiments, researchers observe how flies distribute themselves in containers with light gradients. Time and again, the flies accumulate in the most brightly lit areas. When the lighting conditions are reversed, the flies reposition themselves accordingly.
The innate phototactic response has also been demonstrated in other fly species like the fruit fly Drosophila. Genetic research shows that specific neurons in the fly brain mediate phototaxis. This indicates phototaxis stems from genetically encoded traits that drive flies to seek out light.
Light wavelength influences attraction
While flies are drawn to light generally, they do not respond equally to all wavelengths. Ultraviolet (UV), blue, and green light tend to induce the strongest phototactic responses. Some studies rank UV and blue as the most attractive wavelengths for flies.
In an experiment testing wavelengths of 350-650 nm, researchers found:
| Wavelength (nm) | Fly Attraction Response |
| 350 (UV) | Strong |
| 450 (blue) | Strong |
| 520 (green) | Strong |
| 580 (yellow) | Moderate |
| 650 (red) | Weak |
Shorter UV-blue wavelengths seem to trigger the strongest phototaxis. Yet even longer yellow-red wavelengths elicited some attraction. So the flies still drifted towards those colours despite the weaker response.
Fly photoreceptors likely evolved to prioritize shorter wavelengths relevant for their lifestyle. Shorter blue-green wavelengths are abundant in the outdoors on sunny days. Navigating towards these colours would guide flies towards exits and open spaces during foraging.
Bright sunlight deters house flies
While flies move towards light, extremely intense illumination has an opposite repellent effect. In conditions mimicking direct midday sun, flies exhibit negative phototaxis and actively avoid light.
Direct sunlight can heat a fly’s body to lethal temperatures. Flies prevented from escaping illumination can die within minutes. It appears flies innately avoid sunlight to prevent overheating and desiccation stress.
So there is an upper limit to light attraction. Once illumination becomes excessively bright, flies will retreat to shadier areas as a safety mechanism.
Light combined with other cues lures flies
Light by itself attracts flies. But other environmental cues can synergize with illumination to make an area irresistible to flies.
Smell is one such factor. Flies detect odors using sensory hairs on their antennae. Smells emanating from potential food sources, garbage, and waste act as strong fly attractants.
In agricultural settings, traps combining sun-mimicking LED lights and pheromone lures caught significantly more flies than either stimulus alone. The light first pulls in flies from a distance, then smell keeps them nearby.
Similar synergistic effects have been demonstrated between light and shape. Flies are drawn to vertical objects which can provide resting spots during flight. Traps that couple illumination with vertical dark surfaces can exploit the flies’ attraction to both light and shape.
So while light independently attracts flies, pairing it with supplemental cues enhances the results.
Attraction depends on fly sex and age
Phototaxis in house flies also depends on details like sex and age. Male and female flies display some differing light responses influenced by their biology.
In general, male flies tend to be more strongly attracted to light than female flies. This is thought to relate to mate seeking behavior. Since females congregate in bright areas, males fly towards light in order to find potential mates.
Additionally, younger adult flies exhibit more robust phototactic behaviors than older flies. Older flies with decreased flight ability become less responsive to light gradients. Younger vigorous flies actively steer towards light, while older flies are less capable of phototaxis.
So flies belonging to certain cohorts are more inclined than others to navigate using visual cues. These factors may cause variations in light attraction under different conditions.
Disrupted navigation due to fly eye mutations
Mutant fly strains with eye defects demonstrate the importance of phototaxis for normal house fly navigation.
White-eyed mutant flies lack a key eye pigment required for filtering light. As a result, their vision is impaired and they have difficulties responding to light cues.
In phototaxis assays, normal red-eyed flies accumulate in the brightly lit zone. Meanwhile, white-eyed mutants wander randomly without any consistent light approach. So phototaxis requires fully functional fly eyes adapted to process illumination.
Interestingly, a different fly mutant called Atoni is blind yet retains typical photoresponses. Atoni flies are born without eyes but still instinctively move towards light sources using their peripheral visual system. So multiple pathways mediate phototactic behavior.
Attraction helps flies assess surroundings
For house flies, orienting towards light allows them to better inspect their surroundings. By moving towards illumination, flies gain improved visual information about the environment.
With their fast vision, flies can observe flickering patterns caused by wind and objects moving nearby. Light helps excite these flickers to aid visual perception.
Being positively phototactic keeps flies in well-lit areas optimal for their active lifestyle. They can see potential food, mates, threats, resting spots, and routes of escape more easily when positioned near light.
Trapping flies using attraction to light
The inherent attraction of house flies to light has been exploited for trapping purposes. Light traps are a time-tested way to capture flies and reduce populations.
Different traps operate on the principle of luring flies in with light, then capturing them once within reach. Funnel style traps draw flies into containers, while sticky traps glue flies onto adhesive surfaces.
UV light is especially effective for attracting flies to traps. But visible light wavelengths can also be used.
Traps are ideally installed where fly activity is high, such as near breeding sites or food waste. Light emission guides flies to the trap location.
Yet light on its own may not capture all flies in an area. Supplementing illumination with other attractants like pheromones improves trapping efficiency. This tempts flies that are responsive to cues beyond just vision.
Reducing light to deter house flies
While light can actively draw flies in, minimizing illumination can passively repel them from an area. With fewer light sources available, flies struggle to visually navigate through the space.
Keeping indoor rooms dimly lit and blocking outdoor light entry can make conditions less welcoming for flies. This forces them to use smell and gravity cues instead of vision to orient.
Turning off lights also reduces attraction to windows, an effect additively enhanced by drawing blinds/curtains. With windows darkened, flies are deprived of a glare source they instinctively fly towards.
Phototaxis disruption provides partial control by discouraging fly activity and mobility. Yet it generally does not deter all flies given their other navigational skills.
Conclusion
In summary, house flies do exhibit an innate attraction to light across many settings. This positive phototaxis draws flies towards illuminated areas that improve their vision while providing open spaces for flight. Shorter UV and blue wavelengths are particularly attractive to flies due to their visual systems being optimized for those colours. Extremely intense direct sunlight does repel flies through an overheating avoidance mechanism. While light independently lures flies, combining illumination with supplemental cues creates an irresistible attraction. Traps exploit fly phototaxis to actively capture pests, while light reduction makes environments less conducive to flies. So in many scenarios, house flies are attracted to light through intrinsic phototactic behaviors. Yet there are nuances in their responses mediated by factors like wavelength, intensity, and additional sensory cues. Overall, light attraction is a key characteristic of house flies that aids their visual navigation. Managing illumination can be an effective strategy for controlling fly populations and activity.
