I used a 300mm f/4.0 with a 1.4xTC on a FF D800.
With were around 1m away. This combo doesn't give true macro but with dragon flies that are quite big it works. Not sure of the exact magnification ratio when used with the TC.
The focal length is the key to insects. All macro lenses will be able to go to 1:1 reproduction ratio. This means an item that is 1cm big will cover the 1cm on the sensor. However, lenses with longer focal length will allow you to be further away to achieve that. So for insects a long focal length helps a lot, even if you don't quite get 1:1.
It also took around an hour of standing still waiting until they got used to me and landed I right in front.