I thought they came from dimension-hopping reality warpers.
I think both of the latter are right: ordinary arthropods and invertebrates were corrupted by reality-warping dimension-hopping beings that at one point were referred to as demons (I don't know if that's for lack of a better word, or because this was really long ago and historical memory is mixed with superstition, or they really were demons).
It would be a nice twist though.... (one that the Puella Magi series and i'm sure a few others have done).
Pests actually turning out to be corrupted Flower Knights.... turns out what you were fighting is what you can become...
the first time in two years playing this game...my Master Volume settings have been saved from an update.
Now we just need the Storage update (stackable up to 999 Material Characters) and this version will be above average