[zot]

Something was different about conditions that allowed dragonflies of this size to live and fly. What?

Yes, that is the question. My thought was as follows: Since insects are limited in size by their respiratory system, not their weight, and these six times larger insects have NO [structural] adaptation for their larger size, whatever was different about their environmental conditions had to increase the efficiency of their respiration.

[js1138]

Higher oxygen percentage.

[aruanan]

Since insects are limited in size by their respiratory system, not their weight, and these six times larger insects have NO [structural] adaptation for their larger size, whatever was different about their environmental conditions had to increase the efficiency of their respiration.

Insect respiration is basically a tube from the outside of the body that reaches down to within each couple of cells. If there was a higher percentage of oxygen in the atmosphere, that could support a larger overall form.