Dytiscus dimidiatus - female  Even larger beetle than I saw before (40mm, I think). Here you could see the texture of the elytra (upper wings), that is specific to females of this species. You also could see the green coloration of some exoskeleton parts - it's not real "color", it's effect of iridescence. This effect could be observed, when axis of view makes relatively small angle with exoskeleton surface. That's the reason, why many sources says this beetle is "brown", "olive", "green". Biologists are often not enough familiar with physics.