Good question
See that dark line on the forewing? This is highly-visibly in certain species, such as the "brown" skippers, and some of the fritillaries (especially, Silver-washed and High Brown). This "sex brand" contains special scales, known as androconia, which contain pheromones that are used to attract the female of the species. Such "sex brands" are not found in the female.
Cheers,
- Pete