Common Blue Polyommatus icarus

Sexual dimorphism reaches extremes in this widespread butterfly. Males are unmistakable: brilliant blue wings with black-brown borders and white fringes. Females? They're likely to be mistaken for a different species entirely—brown above with just a blue dusting and distinctive orange spots. Found throughout the Palearctic region and introduced to North America, this is one of the most adaptable butterflies around. The color difference is so dramatic that early naturalists thought males and females were separate species entirely, a testament to the power of sexual dimorphism.