Jigokuchō

are completely black butterflies, except for a small outline of maroon on their lower wings, which serve various purposes.

Overview
They guide the Shinigami through the Senkaimon when traveling between Soul Society and the Human World, thereby saving them from the inconvenience of passing through the Dangai. Jigokuchō are used as messengers between Shinigami, often carrying important orders. Hell Butterflies are taken care of by Shinigami in Seireitei, the latter task being carried out usually by new and inexperienced members of the Gotei 13. Jigokuchō, like the Shinigami themselves, are only visible to spiritually aware beings.