cricket, grasshopper
蟋 centers on a specific insect: the cricket. The grasshopper meaning is a closely related insect, but the primary image is a chirping cricket.
蟋 is a phono-semantic compound: the insect radical 虫 indicates the meaning, while 悉 likely contributes the sound. The exact historical development of the right component is uncertain.
The left side 虫 tells you it's an insect. The right side 悉 looks like a complex tangle of legs and wings, like a cricket's body. Picture a cricket tangled in its own chirping song.
For シツ, imagine a cricket sitting on a leaf, making a sharp 'shitsu!' chirp. The sound 'shitsu' cues シツ while the cricket sits there.
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