Butterflies reproduce the way other animals do - sperm from a male fertilises eggs from a female. Males and females of the same species recognize one another by the size, color, shape and vein structure of the wings, all of which are species specific. Butterflies also recognize each other through pheromones, or scents. During mating, males use clasping organs on their abdomens to grasp females.
Many male butterflies deliver more than just sperm to their mates. Most provide a spermatophore, a package of sperm and nutrients the female needs to produce and lay eggs. Some males collect specific nutrients to produce a better spermatophore in an attempt to attract a mate. Some females, however, don't have a choice -- in some species, males mate with females before they have left their chrysalis or swarm the chrysalis waiting for the female to appear. In most species, males and females look a lot a like, but females often have larger abdomens for carrying their eggs.
This post is part Nature Footsteps Winged meme.