Refers to the arachnid of the order Opiliones, not the crane fly (大蚊).
See also: ガガンボ
I found a harvestman in the garden.
The name likely derives from 座頭 (zatō, a blind masseur) and 虫 (mushi, insect), possibly from the long, thin legs resembling a blind person's cane. The exact origin is uncertain.