actually, they’re not blue.
they’re iridescent and the reason why they look blue is because blue light measures at about 400-480 nanometers and the distance between the slits in a morpho’s scales (which look like mascara brushes) are about 200 nanometers apart. this is the magic distance to make them refract light to blue. if the distance between the slits was 165nm, then morphos would be violet.
obviously, I have an interest in this sort of thing. also i think these are Morpho menelaus menelaus, but that’s just based on what I can see of the black on the margins and on my judgement of the blue colour. they could be Morpho didius, too.
reposting for SCIENCE.