worm, grub, maggot
蛆 centers on small, soft-bodied larvae that feed on decaying matter: worms, grubs, or maggots. The meanings are closely related as different stages or types of such creatures.
蛆 combines the insect radical 虫 with 且, which likely provides the sound. The character specifically refers to maggots or larvae.
The insect radical 虫 on the left hints at a bug, and the right side 且 looks like a pile of wriggling shapes. Picture a mass of maggots squirming together.
For ソ, imagine a maggot crawling on a bar of soap: soap -> ソ, and the maggot leaves a trail on the soap.