Houseflies may enter your home for a number of reasons, but they may be in your windows because they are trying to find the way out of your house. Many of the features that give flies their unusual ability to sense and respond to threats become confounded when flies encounter window glass.
Houseflies have amazingly complex eyes; they have both compound and simple eyes. Their eye structure includes a honeycomb shape and eight thousand lenses. Houseflies can detect movement in 360 degrees, ultraviolet (UV) and polarized light, and they possess color vision. According to the Utah Education Network, the large compound eyes of houseflies allow them to easily detect movement, but since the eyes cannot focus, flies see everything as a blurry image. When flies are trapped in your window, perhaps the reason is because they simply cannot see well enough to escape.
Fly physiology and behavior add to the confusion a fly seems to experience when faced with a window. Houseflies are only active during daylight hours, and flies generally become more active when temperatures rise and when light intensity and humidity increase. A fly that finds itself in a window on a warm, sunny day may experience several of these stimulating factors at once, which may make it reluctant to leave the area and, at the same time, cause the fly to be very active.
Houseflies use light as a means of orientation, depending on light from the sky in particular, to navigate. Houseflies tend to move toward sunlight, so they will collect around doors and windows. Flies caught in a window may be somewhat disoriented. Windows allow UVA light rays to penetrate, but block UVB rays, which may confuse the visual signals of flies or cause them to miscalculate what they are seeing. UVA and UVB rays are ultraviolet light rays emitted from the sun. UVA rays are long-wave rays, while UVB rays are shortwave rays. Both are invisible to human eyes but visible to houseflies.
The behavior of flies around light depends on a combination of factors, ranging from the sex, age, nutritional state and searching activity of the fly to the temperature and visual stimuli present in the environment, according to dissertation research from the University of Gronigen. Flies may initially follow the aroma of food odors, which emanate from and accumulate around windows, then find themselves stuck. The research further found that decreasing light intensities induces a landing response in houseflies. On a day where cloudy conditions cause sunlight to seem to brighten and falter, flies may find themselves repeatedly motivated to land.
The fly you see in your window may not be a housefly at all but a cluster fly. Cluster flies look similar to houseflies but are slightly larger with overlapping wing tips when at rest. They make a buzzing sound and fly in a sluggish manner. These flies enter homes through cracks and gaps, such as those found around windows. The flies become active with warmth as found in sunny windows.