When you see a Monarch butterfly, it's plausible you have no idea whether it is male or female. You realize, too, that a cursory look at a butterfly won't give you enough information to figure it out. If you are discerning, though, you can determine a Monarch’s sex by looking at its wings, abdomen, color or size.
Abdomen shapes differ
The Monarch’s abdomen is in its tail section and divided into eleven portions, which house their genitalia and other organs. Females have a groove on the bottom side of their abdomen, while males don’t.
Claspers are armlike extensions on the rear section of the male’s abdomen. They're used to clasp the female’s abdomen while mating. Females don’t have claspers.
A Monarch’s wings remain the same size from birth to death, with the male’s wings larger and having more bulk than females. The degree of bulk is determined by nutrition from available flowers and weather conditions. Wing bulk is important, because more bulk means more endurance while migrating.
The veins in female’s wings are thicker than males.
Males have a protruding black sac, called an alar, alongside a vein on each side of their hindwing. The black alar is not coloration, but a special cell that manufactures a scent to attract female Monarchs.
Female Monarchs have no alar.
Coloring and Size
In some species, the female Monarchs are darker in color and smaller in stature than the males. This allows them to blend in with milkweed plant leaves, where they deposit their eggs.
The female Monarch lays about 1,000 eggs in her two- to six-week lifespan. The transformation from egg to caterpillar to cocoon to butterfly is fraught with danger, and only a small percentage live.