The most important population — Ephemera

The most important population 1
Unnoticed dwell,
They have a heaven each instant
Not any hell.

Their names, unless you know them, 5
'Twere useless tell.
Of bumble-bees and other nations
The grass is full.
(F.1764/J.1746)
[1] important:: urgent, pressing of life; ephemera (also called may-fly or day-fly) compresses its life in a day.
[3] heaven:: the sky with daylight.
[4] Not any hell:: they often don't see the dark night in their life.
[5] names, unless:: ephemera means originally a) a fever lasting only for a day (OED n.1, 1398); b) one who or something which has a transitory existence (OED n.2, 1751).
[6] useless:: if it has only a short life or transitory existence, then it's useless to tell its name.
[7] bumble bees and other:: a hint on the answer as an insect.
[8] full:: mayflies usually hatch at the same fime for a day or two, and cover all the available surface.

TO THE MAY FLY. Poor insect! what a little day Of sunny bliss is thine! ─ The Poetical Works of the Late Mrs. Mary Robinson (1806)