We love this weird little detour. Most of the images so far have been about the sky and heaven and love. Now, all of a sudden we get a single simile between the skylark and a bug. Granted, it's a pretty cool bug. A glow-worm is an English name for a critter like a firefly, and it's hard not to be amazed by the beauty of a firefly. (Unless you're six, and you put them in a jar, and forget to poke air holes, and then they die. Yeah, we had some traumatic firefly experiences. Thanks for bringing it up, Shelley!)
Line 46: The glowworm itself is hard to see, but the light it sends out is really hard to miss. That's the basic comparison with the skylark, which is invisible but easy to hear. We get some sweet alliteration, too: "glow-worm golden."