Gnats are dancing in the air.

October 4. 

The maples are reddening, and birches yellowing. 

October 4, 2020

The mouse-ear in the shade in the middle of the day, so hoary, looks as if the frost still lay on it. Well it wears the frost.

Bumblebees are on the Aster undulatus, and gnats are dancing in the air.

H. D. Thoreau, Journal, October 4, 1853

The mouse-ear in the shade in the middle of the day, See October 2, 1857 ("There is a more or less general reddening of the leaves at this season, down to the cinquefoil and mouse-ear, sorrel and strawberry under our feet.") See also A Book of the Seasons, by Henry Thoreau, the Mouse-ear

Bumblebees are on the Aster undulatus. See October 6, 1858 ("The Aster undulatus is now very fair and interesting. Generally a tall and slender plant with a very long panicle of middle-sized lilac or paler purple flowers, bent over to one side the path"); October 19, 1856 ("Of the asters which I have noticed since [the 8th], the A. undulatus is, perhaps, the only one of which you can find a respectable specimen. I see one so fresh that there is a bumblebee on it.") and note to October 20, 1852 ("Canada snapdragon, tansy, white goldenrod, blue-stemmed goldenrod. Aster undulatus, autumnal dandelion, tall buttercup, yarrow, mayweed")

Gnats are dancing in the air. See  October 19, 1856  ("Each insect was acting its part in a ceaseless dance, rising and falling a few inches while the swarm kept its place. Is not this a forerunner of winter?") See also A Book of the Seasons, by Henry Thoreau, Fuzzy Gnats (tipulidæ)