These flies usually live outside, where they feed off flowers, fruit, and sometimes tree sap. When seeking shelter for the winter, they can fly distances of more than 2 km. They congregate in great numbers in the walls of selected buildings with the greatest sun exposure. They get inside through structural defects and other available openings. Once there, they cluster together in attics, walls, and other hollow spaces in the structure, which is why they are called Cluster Flies.
These flies are easy to tell apart from House Flies by their behavior. They are very attracted to ultraviolet sunlight and often buzz around in windows. An ordinary electric light can also attract them-they can flit for hours around a lampshade or fluorescent lights.
Their behavior is unpredictable. Sometimes they will walk right on your face and fall into your coffee mug for no apparent reason. Fortunately, they do not breed indoors. In nature they hibernate in hollow trees under the bark as well as in rock crevices.
In colder rooms, these flies can remain dormant right until spring. However, those that have taken refuge in warmer spots can emerge at any point during the winter and cluster in the windows. Between spring and fall, they spend all their time outdoors.