The largest insect known to have existed looked like a gigantic dragonfly. It is not known whether it is a direct ancestor of modern dragonflies but it is more directly related to a griffinfly. The largest of these was the Meganeuropsis Permiana, which could reach a wingspan of 28″ or 71cm. There are many theories as to what caused the creatures to get so big but one is that the early atmosphere on earth had 20% more oxygen than we have now.
How big is that? Well, it’s gigantic. The largest dragonfly that currently exists only has a wingspan of 7.5″ so the ancient relative is nearly four times as large. Even in body mass, it was the largest insect. It was even large than this thing:
In fact, here’s a to scale model of it on a log:
Well, at least it wasn’t a mosquito!