Monarch butterflies may all look the same, but there is a way to tell the difference between a male and a female. Here is the key to identifying them.
Males have thinner wing veins than females do. The males also have two distinct black spots on the lower (hind) wings.
Females have thicker wing veins than males and lack the black spots on the lower wings.
Oftentimes, other butterflies are mistaken for monarch butterflies. Some species have evolved to look like monarchs because it helps them keep predators away (as monarchs are bad tasting and poisonous). So how can you tell what is a monarch and what isn't?
This viceroy butterfly looks a lot like a monarch, but there are many small differences.
First, you can see that the patches on the wings are different. The viceroy has a line that goes across the hind wing, making it look like it has two main sections. Monarchs do not have that line.
The back of the viceroy's body doesn't have any of the white dots seen on the back of the monarch's body. And if you look at it from the side, you can see the viceroy has a few larger spots than the monarch, as well as white stripes along its body. The monarch's body is all spotted, with no striping.
Additional images via Wikimedia Commons. Mating monarchs by forehand.jay. Viceroy from the back by Benny Mazur.
Tracy Fuentes. (2009, December 18). Male and Female Monarchs. ASU - Ask A Biologist. Retrieved December 3, 2020 from https://askabiologist.asu.edu/male-and-female-monarchs
Tracy Fuentes. "Male and Female Monarchs". ASU - Ask A Biologist. 18 December, 2009. https://askabiologist.asu.edu/male-and-female-monarchs
Tracy Fuentes. "Male and Female Monarchs". ASU - Ask A Biologist. 18 Dec 2009. ASU - Ask A Biologist, Web. 3 Dec 2020. https://askabiologist.asu.edu/male-and-female-monarchs
Mating monarch butterflies.