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Gonepteryx rhamni (known as the common brimstone) is a butterfly of the family Pieridae. It lives throughout the Palearctic zone, commonly found across Europe, Asia, and North Africa. Across much of its range, it is the only species of its genus, and is therefore simply known locally as the brimstone. The name "butterfly" is believed to have originated from the brimstone —  which was called the butter-coloured fly by early British naturalists.

The brimstone relies on two species of buckthorn plants as host plants for its larvae; this influences its geographic range and distribution, as these plants are commonly found in wetlands. The adult brimstone travels to woodland areas to spend seven months overwintering until spring arrives, when host plants have developed, and then they return to the wetlands to breed and lay eggs. Both the larval and adult forms of the common brimstone have protective coloration and behavior that decreases their chances of being recognized and subsequently preyed upon.

The adult common brimstone has sexual dichromism in its wing coloration and iridescence; males have yellow wings and iridescence while females have greenish-white wings and are not iridescent. This iridescence is affected by environmental factors.

Geographic range
The common brimstone is a butterfly that can be commonly found throughout the Palearctic zone, from western Europe to east Asia due to its high mobility, which allows it to search widely for new host plant locations and therefore expand its range. While the butterfly has a wide geographic distribution that is larger than that of its host plant and it is quite abundant, its range is limited by its need for the presence of host plants due to the needs of its larval stage, as its range also expands upon the geographic expansion of its host plants.

Habitat
The common brimstone is highly mobile and utilizes various environments for different stages of its life cycle. The butterfly inhabits wetlands during mating and breeding as they provide ideal areas for oviposition due to an abundance of host plants like the alder buckthorn. The common brimstone prefers laying eggs on younger host-plants with late bud-bursts that are isolated from other plants in the area and exposed to both open space and sun. During the winter, adult brimstones travel to woodlands to hibernate, as they provide ideal overwintering sites with shelters such as evergreen foliage and holly commonly utilized. In other seasons, habitat selection is also affected by the abundance of nectar plants as a food source for adult brimstones.

Caterpillar
Larval brimstones appear to only feed on two plant sources: the alder buckthorn (Rhamnus frangula) and the common buckthorn (Rhamnus carthartica). This influences the distribution of the adult brimstone, as the presence of these two buckthorn species is necessary for larval survival.

Adult
Adult brimstones are not specialized nectar feeders, unlike their larval forms, which are specialized for particular host plants. The common brimstone heavily feeds on the nectar of several flowering species: Centaurea jacea, Knautia arvensis and Succisa pratensis. However, brimstones have also been seen feeding on the nectar of Tussilago farfara in April and May and have been recorded gathering nectar from many other species of flowers. Food plant availability is another factor that is important for habitat selection.

Oviposition
There are several ideal characteristics of the particular host plants chosen for oviposition. Adult brimstones lay eggs on the underside of the leaves of the two species of host plants, decreasing their visibility. The high mobility of G. rhamni enables the butterflies to find even the most isolated host plants in an area, which are more ideal for their offspring. Eggs are less likely to be deposited on outlying plants, leading to reduced vulnerability as fewer predators are attracted to these plants. Another factor is damage; undamaged plants demonstrate the absence of other eggs, as brimstone larvae leave holes in the leaves of the plants on which they feed - since predators and parasites are attracted to damaged plants through chemical or visual signals, less damage leads to greater offspring survival. Plants exposed to both sunlight and the open lead to reduced chances of predation and parasitism as well, and are more accessible to adult butterflies. Larvae can also benefit from decreased host plant defenses; juvenile plants and plants with late bud-bursts have less toxins and therefore defense chemicals, as resources are more directed towards plant growth.

Life cycle
The common brimstone is one of the longest-living moths, with a life expectancy of around a year, and has one generation a year. However, the adult brimstone spends a large portion of its life in an overwintering state. The brimstone is highly mobile, feeding and traveling 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 on the underside of buckthorn leaves. The eggs are around 1.3 mm tall, and are spindle-shaped in appearance. The eggs change color over time, initially having a greenish-white coloration, then yellow, and finally brown before hatching.

Caterpillar
The larvae of the common brimstone undergo five instars. The caterpillars have a green coloration 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 coloration makes them difficult to distinguish.

Pupa
Pupation occurs across approximately two weeks. The pupae are between 22.2 to 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 cremaster hook attaches to a silk padding, and a length of silk secures the pupae around its middle. The pupae are primarily a green coloration, 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, near the end of fall. For the next seven months, during the winter, the common brimstone hibernates, remaining inactive until April, where they then emerge and proceed to reproduce and lay eggs. Adult brimstones are long-lived, and so are highly abundant for several months after their emergence from overwintering. The common brimstone has sexual dichromism, with males having a sulphur yellow wing coloration and males having a greenish-white wing coloration. Additionally, males have iridescent dorsal wings that change under ultraviolet light, while females do not.

Migration
The common brimstone undergoes some regional migration between hibernation and breeding areas throughout the year, as seen in the different chemical composition of butterflies across varying seasons and regions. In general, there is movement towards wetlands to reproduce, and after the eggs hatch, develop, and pupate, newly hatched adult butterflies emerge and disperse locally into both woodlands and wetlands to overwinter. Butterflies travel to the woodlands for overwintering, and no mating appears to occur within these habitats. Overwintering also occurs in the wetlands, where the host plant alder buckthorn is abundant, and after emerging from hibernation, adult brimstones that were previously in the wetlands are joined by those that hibernated in woodlands, and all of the butterflies breed and lay eggs.

The environmental conditions of a particular year also affect migration, as seen in the elevational migrations of the common brimstone. Uphill migration is potentially influenced by habitat limitations, such as a lack of the forest cover that is required during overwintering. Brimstones travel to higher elevations for greater forest cover and reduced exposure to higher temperatures during their activities. Downhill migration is influenced by the need for larval resources such as host plants during breeding seasons - the butterflies travel to lower elevations in search for regions containing these plants, with adults commonly returning to the areas where they had been bred due to their long lifespan.

Predators
Like most woodland Lepidoptera, G. rhamni is preyed upon by many species of birds and wasps. Both larvae and adult brimstones fall victim to predation and utilize means such as protective coloration and mimicry to avoid this fate.

Parasites
The common brimstone has two recorded species of parasites: the braconids C. gonopterygis and C. risilis. These two species of parasitoid wasps are completely specialized for G. rhamni, possibly due to the wide distribution of the butterfly and the host plants in its habitats - with a broad presence of its host, the wasps can afford to be host-specific. The wasps are primarily associated with the presence of the food plant Fragnula alnus due to its association with their host.

Protective coloration and behavior
Both the larval and adult common brimstone exhibit cryptic coloration, as both match their habitats in color. Larvae are difficult to see due to their coloration; they match the color of their host plants so well that they can remain in the open undetected. When not eating, the caterpillars remain still in a position alongside the midrib of leaves, making them even more difficult to spot. Adult brimstones are leaf-mimics, as they share similarities in shape, color, and pattern to leaves, allowing them to blend in with their surroundings during vulnerable times like hibernation. When picked up, the butterflies become rigid and hide their legs from view in order to decrease their chances of being recognized.

Genetics of color patterns
The common brimstone exhibits sexual dichromism in wing iridescence and color - male dorsal wings are yellow and iridescent, and female wings are white and non-iridescent.

Pigmentation and structural coloration
The brimstone's wing scales scatter light incoherently due to ovoid-shaped structures called beads that contain pigments. These beads absorb short wavelength light and scatter longer wavelengths outside of the pigment absorption spectrum, such as light in the complementary wavelength range. Through chemical extraction and analysis, two possible pigments have been attributed to the common brimstone's wing coloration. Xanthopterin is responsible for the sulphur yellow color of the male wings since it absorbs in the violet range, and leucopterin was extracted in the white wings of females. This explains the lack of iridescence in female common brimstones, since leucopterin absorbs only in the ultraviolet range, so the wings do not reflect and consequently scatter any ultraviolet like like male wings do. A male-only pattern of coloration is seen exclusively under ultraviolet light, since females absorb on the ultraviolet spectra. However, in males the wing-pattern appears to visually change depending on the position of the ultraviolet light shone onto the wing; at some angles, a male pattern is seen, while at other angles, a female lack of pattern is seen - this is referred to as the "gynandromorphic effect." This demonstrates that the pattern appears to be optical, rather than pigmental, as the effect is only seen at certain angles and distances of light and changes with positions.

The structural coloration of the male dorsal wings is affected by environmental factors; there is an increase in ultraviolet coloration coverage with increasing temperature and precipitation and decreasing latitude. This has been possibly attributed to several factors, such as the greater abundance and quality of resources in areas with those environmental conditions or a better ability to assimilate resources as an indication of male mate quality, as ultraviolet coloration is energetically expensive to develop.

Vision
The common brimstone appears to have an innate preference for certain colors in nectar plants - red and blue inflorescences are common in heavily-used nectar sources in some regions. G. rhamni also has a stronger reliance on visual indications such as color compared to other butterfly species, which rely more on odor.

Olfaction
The common brimstone has an antennal response to the floral scent compounds of nectar plants. Research suggests that there are antennal olfactory receptors for phenylacetaldehyde and the terpene compounds oxoisophoroneoxide, oxoisophorone, and dihydrooxoisophorone, as these compounds elicited some of the strongest electrophysiological responses whether they were presented in natural or synthetic mixes of floral compounds. Additionally, these two compounds are present in the largest quantities in the nectar plants utilized by the brimstone, indicating that scent detection could be important for detecting food sources. This would contribute to more efficient foraging in adult butterflies, as odor could act as a cue for finding and distinguishing nectar plants, allowing more energy to be utilized for other activities such as reproduction.

Diapause
The adult common brimstone overwinters for seven months. While both sexes have similar egg to adult development times, they differ in the times that they reach sexual maturity. The reproductive development of males begins just after pupal emergence, and continues during hibernation, which indicates that males may not be able to reproduce until after overwintering. For females, eggs remain undeveloped as the butterflies overwinter, and no reproductive development occurs until after emergence from hibernation.

The sexes also differ in times of emergence after overwintering. Emergence is correlated with temperature and hours of sunlight; a certain amount of both is necessary for the butterfly to awaken from hibernation and therefore influences when diapause ends. Males emerge earlier than females, as they are more willing to fly in lower temperatures compared to females. Since the common brimstone most closely follows monandrous mating patterns, males may emerge earlier to increase the number of mating chances and therefore reproductive success, as older males have had more time to develop and therefore have a greater advantage. In contrast, females emerge late due to the late seasonal development of host plants such as the alder buckthorn - since these plants are necessary for egg-laying, female emergence is correlated with host plant development.

Conservation
As of 2010, G. rhamni does not appear to have a threatened conservation status according to IUCN standards. However, the butterfly has experienced significant population and distribution reduction in areas such as the Netherlands, where its numbers have declined to the point that based on IUCN criterion, it has reached endangered species status. The causes of this population decline are not fully determined, but there are several possible factors. Since the common brimstone is univoltine, it may have difficulties adapting to changing environmental conditions compared to species with multiple generations a year. For example, there has been a decrease in suitable environments for the butterflies, with open woodland decreasing in favor of more urban areas. Additionally, nitrogen pollution, declining nectar supplies, and rapid ecological changes have been suggested as other hypothetical factors. Concerns have been raised about the possible future increase of this population decline, but the butterfly mostly does not appear to be a conservation concern.