might cause adverse reactions in humans, such as allergies or severe skin reactions.
This is true for every antibiotic used on human beings as well. All of them can give you severe skin reactions or you might develop potentially deadly allergies against them. It might be rarer with the ones we use regularly, I don’t know, but it happens.
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.