Where the shape comes from
Originally a small coiled viper, head raised — the same snake later widened to cover crawling and flying creatures of every kind, insects, worms, reptiles alike, once its own meaning had been crowded out by more specific words. Tripled in the fuller traditional 蟲, it named vermin and insects generally before simplification folded the three back down to one; on its own it still keeps the oldest sense in 蛇, a snake, and names a rainbow in 虹, once pictured as a two-headed serpent arched across the sky.
汉字 filed under 虫
No published character on HSK Guide is filed under this radical yet. 汉字库.