So, if the princess thing made sense to you, maybe this will be a little more surprising. Now the speaker compares the skylark to… a bug. Ew.
That's right, the beautiful singing sky-spirit is now like a worm. Well, to be fair, it's a pretty cool glowing worm.
The English glow-worm is like the fireflies we have in the US, except it doesn't fly. You'd notice them at night, maybe in a damp little valley (a "dell of dew").
See our "Best of the Web" section for more about these cool little critters.
Scattering unbeholden Its aerial hue
These lines continue the theme of secret beauty that's so important to this poem. The speaker is fascinated with things in nature that make beautiful songs or sights, even when they no one is watching (when they are "unbeholden," as he so poetically puts it).
The "aerial hue" connects the color ("hue") of the worm's glow to the sky and the air and the world of the skylark.
Among the flowers and grass, which screen it from the view:
Like the far-away skylark, this little worm makes wonderful things happen—even when it can't be seen itself.