kind of spider with long legs
蠨 refers to a specific kind of spider with notably long legs. Its meaning is narrow and does not extend to other spider types or general concepts.
蠨 combines 虫 (insect) with 蕭, which likely provides the sound. The exact historical development of this character is uncertain, but it was probably created to name a specific long-legged spider.
The insect radical 虫 on the left marks it as a bug, while the complex right side 蕭 suggests something tall and slender, like the long legs of the spider it names.
For ショウ, imagine a long-legged spider putting on a tiny show: show -> ショウ, and the spider's legs stretch out like stage curtains.