The fastest useful split
Cluster flies are large, slow and seasonal at upper windows; drain flies are tiny, fuzzy and tied to wet film.
Do not rely on a single color match. Start with scale and physical form, then check where the sign accumulates and whether a companion clue supports one candidate over the other.
| Feature | Cluster fly | Drain fly |
|---|---|---|
| Scale | Large fly | Tiny moth-like fly |
| Location | Upper window/attic | Drain/wet wall |
| Season | Overwintering emergence | Warm wet source |
| Breeding | Not indoors | Wet organic film |
Map the source by building height and season.
Persistent drain activity may require plumbing inspection.
Open safety guidanceCheck each full evidence profile
Cluster fly
Pollenia spp.
Slow dark flies clustering at upper windows or attics seasonally, without an indoor food or drain breeding source.
Drain fly
Psychodidae
Fuzzy triangular wings, weak short flight and adults resting near one persistent wet-film source.
Why a visual match can fail
Scale can be distorted by a close-up photograph, loose material may be moved by cleaning or airflow, and old evidence can change color or texture. A reliable conclusion uses multiple observations that point in the same direction and preserves contradictory details instead of forcing a match.