Why is the yarenma moth so different from its cousin in appearance? Well, the answer to that is connected to the Willow Kingdom story. When the tribes of the Kokusei's descendants traveled through the Yamato Mountain Range, ever prospering, they nearly hunted the murasaki moths into extinction. These were relatively peaceful and passive, not having any natural enemies, which made them easy prey for the humans.

[...]The reproduction of those moths who were best suited to repell humans was fostered by this, and the traits became more and more pronounced over a long period of time. Over several thousand years, random mutations to their genome lead to their adaptation into the form we know as yarenma moth today: Additional and larger antennae for keener senses and the vaporization of heavy concentrations of moth dust, which itself mutated first to cause nausea in mammals, then to induce sleep and coma; — Rickard Leeuw in " The Codex Riccardium: Magna Magia

This comatose sleep is referred to as Kuikoroshi no Nemuri 食い殺しの眠り ("the Sleep of Devouring"), and no person throughout the history of Aqualon has ever been reported to wake up from it, though very few have ever had the luxury of receiving treatment after falling victim to it.The narcotic dust dispersed by the deadly yarenma moth incapacitates, puts people to sleep, and finally drags them into a comatose state so deep they never wake. Once a mammal has been taken out in this manner, the moth will approach and use its barbed tongue to lick the flesh from the bones of its victim. At some point during this, the animal or person will die due to blood loss or organ damage but never wake up before that.It is believed that the people trapped in the Sleep of Devouring experience terrible nightmares until the moment they die, for their faces are painfully contorted when they have fallen into their coma, an unusual thing to happen for people who have been put to sleep as muscles usually relax in that state.All of this has lead to the people rightfully fearing these terrifying moths, in a way validating the traits they helped grow.