grasshopper, locust
蚱 centers on grasshoppers and locusts, the jumping insects often found in fields.
蚱 combines the insect radical 虫 with the phonetic component 乍, which likely indicates the sound. The character is used specifically for grasshoppers and locusts.
The insect radical 虫 on the left tells you it's a bug, and the right side 乍 looks a bit like a grasshopper's bent legs ready to jump. Together, 蚱 is a grasshopper or locust.
For サク, imagine a grasshopper hopping into a sack: sack -> サク, and the insect lands inside.
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