Male Monarch butterflies have two scent glands and females do NOT have them. You can see one on the butterfly in this photograph... on his wing about 3/4 inch from the left and down about 3/4 inch from the top... alongside one of the dark lines. It is a dark hump that is visible from both the bottom and the top of the wings. Thanks for asking!
Actually I did NOT know this until this set of photographs. When I noticed the spots on some of the Monarch butterflies I looked it up and was amazed... I have been around these fantastic butterflies all my life and had never even HEARD of the reason.