lightning-bug, firefly
蛍 centers on a small glowing insect: a firefly or lightning-bug. The meaning is concrete and specific to this luminous beetle.
蛍 is a phono-semantic compound: 虫 (insect) gives the meaning, and the top part (⺌ over 冖) likely contributes the sound ケイ. The character originally referred to a firefly.
The bottom 虫 is an insect, and the top ⺌ over 冖 looks like a little glowing light with a cover. Imagine a tiny insect carrying a lantern: that's a firefly.
For ケイ, picture a firefly glowing like a tiny letter K in the dark: K -> ケイ, and the firefly's light spells out its reading.
fluorescence
firefly (Luciola cruciata); lightning bug; glowworm
firefly squid (Watasenia scintillans); sparkling enope squid
glow of a firefly
Hotaria parvula (species of firefly)
Aquatica lateralis (firefly species)
flying firefly
sea firefly (Vargula hilgendorfii); seed shrimp
Genji firefly (Nipponoluciola cruciata)
vacuum fluorescent tube
green fluorescent protein; GFP
Auld Lang Syne
fluorescent ink
highlighter (pen)
fluorescent substance
fluorescent paint
fluorescent whitening agent; fluorescent brightener; fluorescent bleach
fluorescent screen
fluorescent dye
fluorescent lamp; fluorescent light