big clam, mirage

蜃 is composed of 辰 (time) and 虫 (insect). 辰 depicts supports that prevent a cliff from collapsing and carries the sense of something temporary, while 虫 broadly refers to all living things and can also mean natural phenomena. Therefore, 蜃 conveys the idea of a natural phenomenon of short duration, and is used to represent an optical illusion caused by the refraction of light. The ancients believed that this illusion was produced by a legendary giant clam, so 蜃 also came to mean a large clam.