Deep below the earth somewhere in Japan, in 1957, miners opened a shaft that exposed dragonflies the size of Toyota hatchbacks, which attacked and killed several of the poor guys. No, wait, that's the opening of Rodan, let's start again.
A good 300 million years ago, giant dragonflies (categorized under the genus Meganeura) buzzed about the ferny swamps in their adult form with wingspans of about a yard, eating other insects and the pre-cursors of salamanders and geckos. They are said to have reached this size due to the much heavier oxygen level at the time.
Recently, mad scientists at Arizona State University have been attempting to grow giant dragonflies, probably to release in mass swarms as assistance to the Japan Self Defense Forces against attack from Godzilla. They had to hand feed juvenile dragonfly in oxygen concentration levels of up to 30% and discovered they grew to adulthood in the higher levels at a much faster rate, while at the same time discovering cockroaches took a lot longer. They plan to keep studying insects reared at different oxygen concentrations, so make sure to stock up on tennis racquets and electrified fencing in case of giant dragonfly attack.