Underworld
Your forehead is in the sun, and you start seeing those creatures with lids for heads and fins for feet again. Where on earth did they come from? They must have come from the underworld. Nothing like that exists in this life.
Imagine the heat of the sun on your forehead as you watch these horrible creatures with lid-heads and fin-feet. There's no doubt that these are creatures from the underworld. You get out of the sun as fast as you can.
めい, みょう
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The sun is unusually hot for the month of May (めい), which must be why those underworld creatures are running around.
You make a mental note to always avoid the sun in May.