Written almost 300 years ago, those lines reveal more than their author's command of metaphor. They show keen observational skill as well.
Glowworm is the name commonly assigned to the larvae of fireflies or light ningbugs.
The term glowworm is more commonly used in Europe where these insects patrol gardens above ground seeking slugs and other prey. In North America most of the larvae of our 50 firefly species live in the soil where they feed on earthworms and millipedes. Only occasional floods force them to bring their dimly shining light to the surface. Thus we usually see only adult male lightningbugs winking on and off as they fly in search of the females who occasionally blink in response from their grassy hiding places below. What Webster so astutely observed is the cold light produced by these insects, a kind of illumination also emitted by a wide range of bacteria, protozoa, fungi, sponges, crustaceans, fish and lower plants as well.