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It may be that the humidity is too much, open the windows more for ventilation, do a big cleaning, and it will be gone.
Black soldier fly larvae? It may be a larva of an insect of the family Diptera, see the life of the black soldier fly.
I saw it when I was a child, and I was scared. My dad just grabbed it away from me. He said, "This thing smells."
If it is foreign, please isolate it with a gauze, if it is an insect in the balcony, it is preferred to manually catch it or put a basin of water on the ground at night, hang a light bulb 10 cm from the water surface, extinguish other lights, use the phototaxis of insects, fall into the water and drown. Secondly, use aerosol insecticides.
It's damp and dark, just clean up the area around the drain.
Didn't see the diagram.
It had to be repaired and reinforced. >>>More