Special Features of Danainae
Males of some danainae species (such as Blue Tiger and Common Tiger) have pouches on their hindwings. They would suck the sap of Crotalaria retusa, transform the chemicals in the sap to produce pheromones, and store them in the pouch which produce scent to attract female butterflies. As such we can determine the sex of the butterfly based on this feature.
Male danainae sucking the sap of Crotalaria retusa:
Male danainae has a pair of yellow spherical structures on the abdomen. This structure is known as a "hair-pencil", which can stretch freely and is used to attract female butterflies with pheromones. In addition to courtship, when a male danainae is being hunted, the hair-pencil can be flashed out to scare predators increasing the chance of escape.
Danainae larvae are covered in bright colors to warn predators that they are poisonous. While for adults there are two types of warning coloration, strong contrasting colors due to pigments and metallic refractive colors.