dragonfly, moon moth
蛉 centers on a specific insect: the dragonfly, and by extension a similar-looking moth. It is not a general insect term but a precise name for these creatures.
蛉 combines the insect radical 虫 with 令, which likely provides the sound. The character was created to name specific insects, particularly the dragonfly.
The insect radical 虫 on the left tells you it's a bug, and the right side 令 looks a bit like a dragonfly's long body and wings. Picture a dragonfly resting with its wings spread like the strokes of 令.
For レイ, imagine a dragonfly landing on a ray of light: ray -> レイ, and the dragonfly's delicate wings shimmer in the sun.
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dragonfly
whip-carrying habenaria (Habenaria iyoensis)
Orthetrum albistylum speciosum (subspecies of white-tailed skimmer)
broad-winged damselfly (of family Calopterygidae)
Calopteryx atrata (species of damselfly)
Epiophlebia superstes (species of living-fossil dragonfly endemic to Japan)
Habenaria sagittifera (species of bog orchid)
Mortonagrion hirosei (species of damselfly)
Calopteryx cornelia (species of damselfly)
Ceriagrion nipponicum (species of damselfly)
Calopteryx atrata (species of damselfly)
dobsonfly (esp. species Protohermes grandis)
hornworm
owlfly (any insect of family Ascalaphidae); owl fly
dragonfly
yellow darter (Sympetrum croceolum) (dragonfly species of East Asia)