The observant among you may have noticed silvery looking lines on the veins of the Silver-washed fritillary in yesterday’s post – those are its sex brands. In these particular frits, the males have four such brands, two on each top upper wing. The brands contain androconial scales, special scented scales that are used during courtship to attract females. As the Woodland Trust website explains, Silver-washed fritillary ‘courtship is an aerobatic spectacular: the female flies in a straight line while the male loops the loop around her, before showering her in a confetti of scent scales’.
Silver-washed fritillaries are not the only butterflies to have sex brands: they can also be found, for example, in Large and Small skippers (below left and right, respectively). The Learn about Butterflies website (which has a much more detailed explanation for those who are interested in the nitty gritty of butterfly anatomy) explains, the dark diagonal marks on the skippers’ wings
are composed of hundreds of androconial scales. These disseminate pheromones that can be detected by females during courtship. As the male ages the strength of his pheromones diminishes, thus by analysing the strength of the pheromones a female can assess the age and virility of a potential mate.
The more I find out about butterflies, the more fascinated I become.