in an overwintering state. The brimstone is highly mobile, feeding and travelling to regions ideal for hibernation during the late summer and fall, and returning to regions ideal for mating and egg-laying during the spring. Egg Adult common brimstones lay eggs singly on the underside of buckthorn leaves. The eggs are around 1.3 mm tall, and are spindle-shaped in appearance. The eggs change colour over time, initially having a greenish-white colouration, then progressively darker shades of yellow, and finally brown before hatching. Caterpillar The larvae of the common brimstone undergo five instars, initially having a length of 1.7 mm in the first instar and reaching up to 34.9 mm in length when fully grown. The caterpillars have a green colouration with white hairs and dark tubercules across its length. When they first hatch, they move to the top side of the leaves and eat them, leaving characteristic hole patterns in their host plants. During the day, they feed and then rest in the open, lying still on the midrib of leaves, where their colouration makes them difficult to distinguish. Pupa Pupation occurs over approximately two weeks. The pupae are 22.2-23.8 mm in length and have the appearance of a curled leaf, with pointed ends and bulges in the middle. The pupae are secured to stems and leaves using silk; a cremastral hook attaches to a silk padding, and a length of silk secures the pupae around its middle. The pupae have a primarily green colouration, but right before adult emergence for males, the wing areas turn yellow. Adult Adults emerge during the summer, from June to August, and continue to feed until September. The common brimstone hibernates for the next seven months of winter, remaining inactive until April, where they then emerge and proceed to reproduce and lay eggs. Adult brimstones are highly abundant for several months after their emergence from overwintering. The common brimstone has sexual dichromism, with males having a sulphur yellow wing colouration and females having a greenish-white wing colouration. Additionally, males have iridescent dorsal wings that change in colour and appearance under ultraviolet light, while females do not. Both males and females have orange spots in the discoidal cell of each wing, pink head and antennae, and a thorax covered in white hair. Migration The common brimstone undergoes some regional migration between hibernation and breeding areas throughout the year, as seen in the different chemical composition