What must be done, to prevent the hatching?
Must the Arthron/Triton die BY the fire or only while burning.
On normal maps usually I ignited the enemy and killed it with a SR, no hatching.
In the last mission I killed a burning Arthron with the Scorpion, but the Umbra hatched nevertheless.
Other than fire, I use paralysis weapons on the arthron rather than killing it.
Depending on map and situation, one option is to remove the arthron’s weapons so he will run off the field. Doesn’t work on nest etc as they don’t leave those maps.
Mind control also works. You can also mind control the umbra before it hatches. They have low resistance to control.