"Isn't the parasitic ability something the Wet Nurse researched and developed?" You couldn't help but ask.
"Flandre" shook her head, explaining to you that the parasitic ability was gained by the Wet Nurse breaking down genes from a portion of unique Mutated creatures.
Parasites are by original nature a capability of Mutated creatures, including the insect-like ones in their world...
At the time, the Wet Nurse did not decode the life gene of the Firefly, and her grasp of parasitic techniques was not complete. The birth of a semi-finished product like "Jinmu" was already a major breakthrough.
It was only after she embraced the Firefly with her vows broken that she could master a more complete parasitic technique, and then give you the "Perfect Parasite Gene".
Firefly is the origin of all life genes in this world, and only with the grace of the Firefly can Jinmu master the parasite gene that is closest to the original source.