What is a key distinguishing feature of the face fly compared to the horse fly?

The face fly is unique in that it has the ability to overwinter in wall voids, which is a critical survival adaptation. This means that during colder months, these flies can find refuge in protected areas such as cracks or spaces within walls, allowing them to survive adverse weather conditions. By being able to overwinter effectively, face flies can emerge and continue their lifecycle when favorable environmental conditions return.

In contrast, the other characteristics mentioned do not apply to face flies or do not distinguish them distinctly from horse flies. For instance, the larval stages of these flies do not significantly differ in duration; both types generally have similar development timelines. While it is true that face flies may be more prevalent in southern regions, this is not an exclusive differentiator, as horse flies can also inhabit those areas. Moreover, face flies are not necessarily larger or darker than horse flies, as their sizes and appearances can overlap considerably, making this point less definitive. Thus, the ability to overwinter in wall voids stands out as a key distinguishing feature of the face fly.