Chapter 33 of the Jigokuraku manga.
Gabimaru and Gantetsusai watches as the two Dōshi's summon a swarm of their respective insects. Before Gabimaru could strategy, Gantetsusai steps in an cuts down the butterflies and states that he seeks revenge on the Butterfly Dōshi for causing him to lose his left hand. He lunges in for another attack but is hit by and invisible attack by the Centipede Dōshi. Gabimaru and Gantetsusai defend themselves from the Dōshi's attacks but accidently attack each other, causing them to bicker. The Centipede Dōshi asks as to why they wanted to protect Mei despite not having any connections with her. Gabimaru wonders himself as to why he has changed ever since he landed on the island but comes to accept this new behavior, stating that he was repaying a debt. He then asked why they wanted to capture Mei. The Centipede Dōshi tells him that Mei is a key component to their training in the ways of Tao. He explains to them the five training methods and one of them called Bōchū Jutsu involves sexual intercourse with a partner to join the two chi's, Yin and Yang, together. He then goes on to tell them that Lord Tensen possess the two energies and are training in Bōchū Jutsu to create the ultimate Tao. The Dōshi states that they have Yang energy and in order for them complete their training they require Mei's assistants since she possess Yin energy. Explaining her backstory, Mei was considered to be an inferior being by Lord Tensen due to possessing only one energy, following her expulsion from the palace and giving her a choice between death or assisting in the Dōshi's training. Gabimaru becomes shocked after learning that she was left with a scar for this purpose. Gantetsusai becomes disgusted with this information and tells Gabimaru that he will be fighting the Centipede Dōshi as well but is shocked to see him enraged. He steps in and angrily tells them to stop with their rude explanation of Mei. The Dōshi's transform with their insects and declare that they will kill them on the spot. Gabimaru and Gantetsusai stand behind Mei and prepare to fight back.