In Chinese mythology, dilong 地龍 "earth dragon" is one of many types of -long 龍 dragons such as shenlong 神龍 "divine dragon" and huanglong 黃龍 "Yellow Dragon". Since dì 地 "earth; land; soil; ground" semantically contrasts with tian 天 "heaven; sky" (e.g., tiandi 天地 "heaven and earth; universe", see Tiandihui), the dilong is paired with the tianlong 天龍 "heavenly dragon". Chinese dragons were supposedly able to fly, and thus were considered celestial creatures rather than terrestrial ones like the "earthbound" dilong. Two other exceptions are panlong 蟠龍 "coiled/curled dragon; a dragon that has not ascended to heaven" and tulong 土龍 "soil/earth dragon", which refers to the tuo 鼉 "Chinese Alligator" (cf. Japanese mogura 土竜 "mole").
Dilong first occurs in the mid-7th century CE History of the Southern Dynasties biography of Liang dynasty Admiral Wang Sengbian 王僧辯 (d. 555 CE). It says witnesses saw lianglong 兩龍 "two/paired dragons" that ascended into the sky, and this dilong "earth dragon" leaving Liang territory was interpreted as a portent of their defeat in 550 CE. Ronan and Needham cite another context in Wang's biography that says his boat had shuanglong 雙龍 "two dragons" on the side, which they construe as a "literary emendation" for shuanglun 雙輪 "two wheels" describing an early paddleboat.
Dilong or dilongzi 地龍子 "earth dragon child" is an elegant name for the "earthworm; worm", which is usually called qiuyin 蚯蚓. "Long 龍 is employed in Chinese zoological nomenclature in much the same way that English dragon is used in dragonfly or dragonfish". First, "long names lifeforms thought to resemble dragons" (e.g., hailong 海龍 "sea dragon" "sea otter; pipefish" or longluozi 龍落子 "dragon fall child" "seahorse"); second, "long 龍 is closely associated with dinosaurs" (e.g., oracle bones were originally called longgu 龍骨 "dragon/dinosaur bones").
Dilong first means "earthworm" in the Qixiu Leigao 七修類稿 written by the Ming dynasty scholar Lang Ying 郎瑛 (1487–1566 CE). The 1578 Bencao Gangmu pharmacological entry for qiuyin 蚯蚓 "earthworm" lists alternate names of dilong and tulong 土龍 (see above). Li Shizhen notes these names derive from the myth that earthworms (like dragons) can create yinqing 陰晴 "cloudy and clear; unsettled weather".
Dilongsan 地龍散 "earth dragon powder", or Di Long, is used in traditional Chinese medicine. It is prepared from the abdomen of the Red earthworm, Lumbricus rubellus, and has many purported medicinal uses.
