small cicada with a square head
螓 refers to a specific kind of small cicada noted for its square-shaped head.
螓 combines 虫 (insect) with 秦, which likely provides the sound. The character was created to name a particular type of cicada.
The insect radical 虫 on the left signals a bug, and the right side 秦 suggests a square-headed shape. Picture a tiny cicada with a head as square as a stamped seal.
For シン, imagine the thin, high-pitched shin-shin sound of a small cicada: shin -> シン, and the square-headed insect sings.