They started to appear around the middle of Demon King Avel's reign and they started to breed. Initially, the Gods didn't know what to do with these mysterious creatures until it became apparent that they had no Souls and, subsequently, no capability for thoughts and feelings.
Taking these things into account, the Gods decided that these creatures would be allowed to exist as side decoration animals, as they don't really have much of an impact on the global system.
The category of Insect is incredibly broad, it can include land animals, plants, bugs, fish and other assorted organisms, the only common theme amongst Insects is that they are non-sentient, unintelligent beings with no thoughts, hopes, feelings or dreams.
This is an Insect that has a soul. Although they are born soulless creatures, Insects can in fact gain souls and their associated emotional benefits by eating the unhatched eggs of a Gal Monster. This takes the soul away from the unborn child and imbues the Insect with it instead.
This is an Insect that has been bound to the body of an Insect User, a human that can use Insect's abilities for weaponry in exchange for housing the Insect in their body. This process fuses the soul of Insect and User, allowing the Insect to gain its own personality and emotions.
Insects from the world of Evenicle are highly similar to Insects from The Continent, varying primarily in terminology and origin. The term "Insect" refers to all soulless beings in the World of Eve, and so applies to all animals that are not Human, Kalar, Poppin, Dragon. The term also does not apply to Monsters. While the World of Eve is rumoured to have an equivalent to the Holy Beasts, the creation of animals - and thus of Insects - is instead credited to Mother Eve.
There are no known Insect Users in the World of Eve.