Polygonia c-album
Polygonia c-album, or the comma, is a food generalist butterfly species belonging to the family Nymphalidae. The angular notches on the edges of the forewings are characteristic of the genus Polygonia, which is why species in the genus are commonly referred to as anglewing butterflies. Comma butterflies can be identified by their prominent orange and dark brown/black dorsal wings.
Both the larval and adult stages exhibit protective camouflage, mimicking bird droppings and fallen leaves respectively, which reduces predation. The pupae are also cryptic, resembling shriveled leaves. During the later stage of development, the larvae also develop strong spines along their backs. The species is commonly found in Europe, North Africa, and Asia, and contains several subspecies. Although the species is not migratory, the butterflies are strong fliers, resulting in an open population structure with high gene flow and increased genetic variation.
Description
The outer margins of the wings are strongly and irregularly dentate, excavated and angulated.The upper side of the wings has a bright orange color, decorated with brown marks and light spots on the edge. The reverse is marbled with brown. Folded, the butterfly looks like a dead leaf. The hindwings have on the reverse side a white spot usually in the shape of a comma.
The sexual dimorphism is slight and concerns the intensity of the coloration, the silhouette and the size, the male having a wingspan of 22 to 24 mm. and the female of 25 to 26 mm. The seasonal dimorphism is more marked: the first generation has the upperside fawn orange and the underside brown-gold and the hindwing bears distally a broad dark red-brown area in which is situated a row of light brown hastate spots. The underside is dark, being either unicolorous or prominently marmorated, while the second generation has a more red upper and dark brown underside. In the summer form the wings are less dentate, and the hindwing has a narrow dark submarginal band, near which stands a row of light lunules proximally bordered by a band of brown arcs. The underside is of a paler color, being less distinctly — sometimes, however, very prominently — marmorated and shaded.
Taxonomy and phylogeny
The comma belongs to the family Nymphalidae, the largest family of butterflies with 13 subfamilies. Within the genus Polygonia, a sister-group relationship between P. c-album and P. faunus is strongly supported by larval development analysis and synapomorphies. In both species, the adults and larvae have similar polyphagous habits. The genus Polygonia is also closely related to the genera Kanisha and Roddia, each containing a single species: K. canace and R. l-album.Geographic range and habitat
The comma inhabits areas including Europe, North Africa, and Asia. It is primarily a woodland butterfly, living in low-density forests with sunshine and moist soil. Specifically, the species is commonly found in the woodland, country lanes, and garden areas of Norway, Sweden, and Great Britain. As a food generalist, or polyphagous species, comma butterflies can feed upon a variety of host plants, leading to widespread ranges across continents. In response to climate change, they are also undergoing range expansion.Subspecies
The following subspecies are found in the indicated parts of the comma's range:P. c. c-album EuropeP. c. imperfecta North AfricaP. c. extensa western China, central ChinaP. c. kultukensis TransbaikaliaP. c. hamigera Ussuri P. c. koreana KoreaP. c. sachalinensis SakhalinP. c. asakurai TaiwanP. c. agnicula NepalFood resources
Larval host plant preferences and selection
For comma butterflies, food resources consumed during development are the primary source of nitrogen and protein during adulthood. Because they feed exclusively on plants, making them a phytophagous species, the quality of plants upon which the larvae feed is strongly correlated with their future fitness. The larval form is often divided into five developmental stages known as instars. Although during the first three instars larvae are observed to remain almost entirely upon the underside of leaves, the fourth and fifth instar larvae are more active in obtaining food resources. The later instar larvae are specialized feeders and favor several host plants during the larval stage: Urtica dioica, Ulmus glabra, Salix caprea, R. uva-crispa, and Betula pubescens. While pupal weight and overall larval survival rates are similar among larvae regardless of host plant, the larval development times differ significantly. As a result, larvae prefer feeding on plants that allow them to develop in the shortest amount of time. Larvae reared on U. dioica demand the shortest development time and is thus favored over other plants. On the other hand, B. pubescens is at the bottom of the host plant preference hierarchy. Favoring plants in the family Urticaceae is speculated to have originated from the species' ancestors, providing an explanation for larval preference for U. dioica. Within the U. dioica plant, larvae are not shown to differentiate between high quality and low quality nettles, a pattern expected of a polyphagous species.Reproduction and life history
Mating
Mating system
Comma butterflies have a polyandrous mating system where females mate with multiple males to receive the necessary amount of sperm to fertilize their eggs. The polyandrous female distributes her matings equally over her lifetime, so males' mating success increases proportionally to their lifespan. The mating success of both sexes is correlated to the duration of an individual's life, so no difference in mortality rates is observed between males and females.Female mate choice
Females exercise mate choice before, during, and after mating and can distinguish between males who were reared on high-quality versus low-quality host plants. The ability to recognize adults reared on higher quality host plants is selected for because males fed better plants during development provide superior nuptial gifts. In comma butterflies, nuptial gifts are edible spermatophores containing spermatozoa and nutrients. When comparing the two common host plants U. dioica and S. caprea, females preferentially choose to mate with males reared on U. dioica, because these males have higher protein content and increased spermatophore production.Females preferentially mate with males which provide larger investments, in the form of nuptial gifts. When females mated with males with higher-quality nuptial gifts, they not only allocate more resources to egg production but also use the resources to improve their own reproductive success. The investments can be used to increase female life expectancy, female maintenance, and future reproduction. During each mating, males allocate a constant amount of investment towards each nuptial gift, indicating that male mate choice does not play a role in allocation of resources.
Oviposition
Females recognize and select a host plant carefully before laying their eggs upon it, generally favoring host plants where larval development time is minimized. Akin to the preferred host plants for larvae, females prefer plants in the order Urticales. Despite the overall preference for plants leading to short larval development, host plant preference variation between females exists. Although the partiality for certain plant species appears to be inherited across populations, the pattern is not significant within a single population. This pattern of deviation results from the open population structure with high gene flow.Parent-offspring conflict
In theory, females would prefer host plants where their offspring performance is maximized, and the larvae would benefit from being able to feed on the best resources nearby their hatch site. However, this is not always observed in nature due to external factors such as predators, parasites, and pathogens. Instead, there is a trade-off between female host plant preference and larval fitness in many species of butterfly. In P. c-album, instead of accepting the host plant that the female selected, first instar larvae leave their hatch site in search of alternative food sources. Larvae that stay on the inferior host are not only smaller, but also have lower survival and growth rates.Egg mass
Unlike female host plant preference, egg mass is not shown to be sex-linked. Instead, egg mass is most likely controlled by additive autosomal genes, where the egg sizes of offspring are intermediate compared to its parents. The type of host plant chosen during the larval stage is not correlated with their offspring's egg mass, indicating that egg size is not related to fitness.Life history
Egg
Females lay their eggs on a variety of host plants, preferring those that minimize larval development time. Unlike some butterflies who lay their eggs in batches, comma females often lay their eggs singly. After each egg is laid, the female scouts out other possible host plants before determining the site of her next egg. The eggs are green when first laid, and gradually turn yellow and ultimately grey before hatching, which generally takes four to five days. Although the female can allocate more resources into egg production based on the nuptial gifts received by mates, the total number of eggs laid or the mass of the eggs are altered based on the host plant. A lack of correlation suggests that neither egg quantity nor egg mass indicate future fitness for the offspring.Larva
The larval period is separated into five distinct stages or instars. During the first three instars, the comma larvae have a cryptic appearance to avoid detection while they primarily stay on the underside of leaves. Fourth and fifth instar larvae search for food more actively. However, the beginning of the fourth instar also marks the development of black, white, and orange patterns. To avoid predation despite its conspicuous appearance, the larva develops strong spines along its body. The larvae have a continuous white marking along their backs to mimic bird droppings. In its final instar, the white colouration disappears but the spines persist.Three possibilities describing why spines may develop during later instars are as follows: smaller larvae cannot sustain the spines, larger larvae benefit more from spines as their predators shift from invertebrates to vertebrates, or because an effective spine pattern cannot be achieved upon the surface area of the smaller larvae.
Aside from the formation of spines, no other defence against predators appears to be present. While fourth and fifth instar larvae are rarely preyed upon by the same predator, removal of the spines leads to repeated predation, indicating that no chemical defence mechanism exists to deter enemies.