Butterflies and their close relatives moths are insects belonging to the order Lepidoptera. This name is derived from Greek and means scaly winged.
The adult butterfly has four wings which are covered in thousands of overlapping colourful scales, visible under a microscope. These scales and their colours on the upper as well as underside of the wings create patterns that may allow us to distinguish between males and females among many species as well as to recognise seasonal changes in quite a few. Most importantly, the patterns created by scales provide us with visual information that enabling us to identify butterfly species. When you observe butterflies you will find that the underside of the butterlfy wings tends to blend into the background, while the upper side often serves to attract a mating partner.
Here is an example of how the different looks of one species can help with gender identification. Colours and patterns on the upper wing side of the Varied Eggfly (Hypolimnas bolina) provide the reason for the common name.
Male (on left) and female Varied Eggfly (Hypolimnas bolina) - Images by JG-Jutta Godwin and SA-Sylvia Alexander
Upper and underside of butterflies can differ substantially in pattern and colour as the example of the Indigo Flash (Rapala varuna) below shows.
Indigo Flash (Rapala varuna) - Images by SA-Sylvia Alexander, MB-Milind Bhakare-CC BY-SA 4.0
Some butterflies change their wing patterns according to season. The Evening Brown demonstrates that perfectly.
From left: Evening Brown (Melanitis leda) in December, February and May - Images by SA-Sylvia Alexander