So...ok, I'm missing something, then. How does the population get energy into the system, then? They don't photosynthesize, somebody has to eat SOMEthing at SOME point, or they won't have any energy at all for.....activities.
What happens if we put them on IV? How long will they last? Will they go full Mr. Meeseeks? What are you doing over there, scientist?! We need answers!
A lot of insects live for years as their "larval" or nymph form and then come out into the more recognizable "adult form" just to mate and die. In a lot of ways their larval form is their actual form with their adult lifestage being just a flash in the pan to continue the species.
Cicadas can stay underground for up to 17 years for example and dragon fly larva live 2 years just to come out of the water and grow wings for a few months.
Good question, it all actually comes from the great predecessor moth who imbued its children with the energy needed to carry on for generations. This singular imbuement of life has carried them for the last four thousand years, and it is unknown when it will run out. When that time comes, the Luna moth will be no more.