How to distinguish the sex of a monanrch

You can readily distinguish the sex of a monarch butterfly by looking at its wings.

Comparing males to females, males...

• are slightly larger than females,
• are slightly duller in color than females,
• have thinner black wing veins than females,
• and have an enlargement in a vein on the rear wing that looks like a black dot.  This is the scent gland that may help attract females, although its exact role in mating is unknown.

In this photo, the male is above the female. =>

There are also differences on the abdomen.  Males have small appendages called claspers on the underside, at the tip...

...while females
have a cleft on the underside, slightly to the right of the tip in this photo.  =>

 

Males tend to fly about more rapidly and aggressively, while females usually fly more slowly and close to the foliage (looking for milkweed to lay eggs on). 

Males may congregate in certain areas where they look for females, while females are more widely dispersed.  These areas are not defended territories... rather they are more like "pick-up" places.

Determining sex can be tricky, especially in the wild...

• It's hard to distinguish the scent gland on the male when seen from below or when the wings are folded.  You can reliably see it only on the upper side of the rear wing.
• Color differences aren't reliable.  While females are often darker and richer in color, older butterflies lose their colored scales.  Hence an older female can appear "washed out" compared to a younger male.
• While on the average females are larger, there are large individual differences in size.  Some males can be larger than some females, just as in humans.

Top, male. Bottom: female, which is less colorful because her scales have worn off.

So, the only really dependable method is to look on the butterfly's abdomen for the claspers (male) or indentation (female).

More on behavioral differences between males and females.