What is a defining physical trait of the cluster fly when resting?

The cluster fly is characterized by having its wings folded over one another when at rest, which is a distinctive feature that helps to identify the species. This resting position allows the wings to neatly lie flat along the back of the insect, which is typical of many fly species, aiding in their aerodynamic profile.

In contrast, other options do not accurately describe the physical characteristics of cluster flies when they are at rest. For example, if the wings were spread out, it would not correspond to the typical resting posture of the cluster fly. Additionally, while some insects may have hairs or other prominent features, the key identifier in this case is the arrangement of the wings. The presence of large visible antennae does not pertain specifically to the resting state of the cluster fly and can vary significantly among other types of flies. Thus, recognizing the unique wing positioning is essential for effective identification.