It’s called a pseudopupil. It’s the part of the compound eye that’s collinear with your line of sight. As you said, the light is absorbed there and registered by the dragonfly. The surface of the rest of the eye is at an angle and reflects light from the surrounding, which makes it look brighter.

This is worse than Pompeii. The ancient Roman city wasn’t buried under lava, but under ash, rock and boulders. While that is still very deadly, it isn’t nearly as destructive as lava. That’s why we can still experience Pompeii architecture, art and culture. Excavating this road, covered in solidified lava, is extremely difficult and costly. Just go over it, once it’s cooled.