Jigokuchō

are black butterflies used by Shinigami to serve a variety of purposes.

Overview
The bodies and wings of Jigokuchō are completely black, except for a small outline of maroon on their lower wings. 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 also used as messengers among Shinigami, often carrying important orders. Hell Butterflies are taken care of by Shinigami in Seireitei, this task usually being carried out by new and inexperienced members of the Gotei 13. Jigokuchō, like the Shinigami themselves, are only visible to spiritually aware beings.