chrysalis, pupa
蛹 centers on the dormant transformation stage of an insect: a chrysalis or pupa, the protective casing where metamorphosis occurs.
蛹 combines the insect radical 虫 with the phonetic component 甬, which likely contributes the sound and may relate to the idea of emergence or a container.
The insect radical 虫 beside 甬, which looks like a container with a lid, suggests an insect enclosed in a protective case: a chrysalis.
For ヨウ, imagine a yo-yo hanging from a branch, and inside the yo-yo a pupa is transforming: yo-yo -> ヨウ, and the insect is dormant inside.