The Last Dance Of The Summer Shower Gnats

It happens every year.  They come from I know not where.  They dance when I cannot see them.  Then they die.
In the basement shower.

It's a mystery to me.  Every year, these little gnats -- or gnat-like insects -- accumulate in small piles in the basement shower.  I never see them alive.  I never find them on other bathroom fixtures in the basement.  I never find them in the upstairs bathroom.  But I have to clean them out of the downstairs shower once a week or so.
As soon as the autumn arrives, however, their lifeless bodies will disappear for months, only to return when the heat becomes roasting outside again.
I have absolutely no clue how they get in, how they live, or why they choose to die in that particular shower year after year.  I can only assume that the basement shower was accidentally built upon an ancient and revered path of the great migration pattern for the species.
Wonders never cease in our world.
And now, I need to stop writing and go sweep up some gnats.