Míng

It names a crop-damaging moth larva.

The Spring and Autumn Annals, an ancient chronicle, records 螟 as a crop pest. The Shuowen Jiezi, an early Chinese character dictionary, says it eats into grain plants. 虫 marks an insect; 冥 gives the sound.

Why does 螟蛉 mean an adopted child?

It became a traditional word for an adopted child.

The Book of Odes, an early Chinese poetry collection, describes a wasp carrying a 螟蛉 caterpillar. Traditional readers thought the wasp raised it as its own, so 螟蛉 became a metaphor for an adopted child. Some solitary wasps carry caterpillars to feed their larvae.