Insect Imitation's latest wording is "
Tribute 1 monster. Special Summon 1 Insect-Type monster from your Deck whose Level is 1 higher than the Tributed monster's."
One important thing to note here is that, when a monster on the field has it's level changed by something like
Primitive Butterfly's effect, and is then tributed off to activate something, that monster's level resets back to it's original level once it leaves the field, so when you begin to apply the part of the effect of Insect Imitation that special summons, it uses the new level of the monster (which would be the original level in this case, not the level it had prior to being tributed).
In general, if an effect relied on the level the monster had on the field just prior to being tributed, it would refer to the level the monster had specifically on the field, like how
Cupid Pitch's effect is worded to say "If this card is Synchro Summoned: Increase or decrease this card's Level by the Level the Tuner used as material
had on the field," otherwise, it would refer to the monster's new level
after it left the field which would usually be it's original level.