In all insects, oxygen travels through tubes to reach interior cells. The larger the insect, the longer the tubes must be. To see if this would be a problem for a large insect, we'll do a virtual experiment that shows how oxygen moves through tracheae of different lengths.
The tubes below represent the tracheae of three dragonflies. In this model, each tube supplies a gut cell with oxygen the larger the dragonfly, the longer the tube. The blue dots represent oxygen molecules. Click on the Go button to see how oxygen moves through the tubes.
The gut cell in the biggest dragonfly is not doing too well because it is not getting enough oxygen. There is a limit to the length of tracheae (and thus to the size of the dragonfly) that can provide every cell with sufficient oxygen.