The average wingspan of both males and females is 5.1 cm (2 in), although males tend to be slightly smaller than females. Furthermore, males possess a row of grayish-brown scent scales on their forewings that is absent in the females. Females have brighter and more distinct markings than males. The subspecies P. a. tircis is brown with pale yellow or cream spots and darker upperwing eyespots. The subspecies P. a. aegeria has a more orange background and the hindwing underside eyespots are reddish brown rather than black or dark gray. The two forms gradually intergrade into each other. Subspecies P. a. oblita is a darker brown, often approaching black with white rather than cream spots. The underside of its hindwings has a marginal pale purple band and a row of conspicuous white spots. The spots of subspecies P. a. insula are a tawny orange rather than a cream color. The underside of the forewings has patches of pale orange, and the underside of the hindwing has a purple-tinged band. Although there is considerable variation with each subspecies, identification of the different subspecies is manageable.
The morphology of this butterfly varies as a gradient down its geographic cline from north to south. The northern butterflies in this species have a bigger size, adult body mass, and wing area. These measurements decrease as one moves in a southerly direction in the speckled wood's range. Forewing length on the other hand increases moving in a northerly direction. This is due to the fact that in the cooler temperatures of the northern part of this butterfly's range, the butterflies need larger forewings for thermoregulation. Finally, the northern butterflies are darker than their southern counterpart, and there is a coloration gradient, down their geographical cline.
The speckled wood occupies a diversity of grassy, flowery habitats in forest, meadow steppe, woods, and glades. It can also be found in urban areas alongside hedges, in wooded urban parks, and occasionally in gardens. Within its range the speckled wood typically prefers damp areas. It is generally found in woodland areas throughout much of the Palearctic realm. P. a. tircis is found in northern and central Europe, Asia Minor, Syria, Russia, and central Asia, and the P. a. aegeria is found in southwestern Europe and North Africa. Two additional subspecies are found within the British Isles: the Scottish speckled wood (P. a. oblita) is restricted to Scotland and its surrounding isles, and the Isles of Scilly speckled wood (P. a. insula) is found only on the Isles of Scilly. P. a. tricis and P. a. aegeria gradually intergrade into each other.
The eggs are laid on a variety of grass host plants. The caterpillar is green with a short, forked tail, and the chrysalis (pupa) is green or dark brown. The species is able to overwinter in two totally separated developmental stages, as pupae or as half-grown larvae. This leads to a complicated pattern of several adult flights per year.
Territorial defense involves a male flying or perching in a spot of sunlight that pierces through the forest canopy. The speckled wood butterfly spends the night high up in the trees, and territorial activity commences once sunlight passes through the canopy. The males often remain in the same sunspot until the evening, following the sunspot as it moves across the forest floor. The males often perch on vegetation near the forest floor. If a female flies into the territory, the resident male flies after her, the pair drop to the ground, and copulation follows. If another species flies through the sunspot, the resident male ignores the intruder.
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However, if a conspecific, a male of the same species, enters the sunspot, the resident male flies towards the intruder almost bumping into him, and the pair fly upwards. The winner flies back towards the forest floor within the sunspot, while the defeated male flies away from the territory. The pattern of flight during this encounter depends on the vegetation. In an open understory, the pair fly straight upwards. In a dense understory, this flight pattern is not possible, so the pair spiral upwards.
In most of these interactions, the conflict is relatively short, and the resident male wins. The intruder most likely backs down as a serious confrontation could be costly, and there is an abundance of equally desirable sunspots. However, if both males believe they are the "resident" male, the conflict escalates. If a previous owner of the sunspot tries to reclaim his territory after he has left for mating, a longer and more costly fight ensues. In these serious fights, the winner of the contest is not predictable.
The abundance of territorial behavior depends on the environmental conditions. At the beginning of the mating season, fights over ownership of a sunspot territory are lengthy. The duration of the conflict quickly decreases during a period of two weeks. This pattern is correlated with the progression of the season, as temperature and male density rise. Sunspots are more attractive when temperatures are low, as they provide the warmth needed for higher levels of activity. As male density increases, it becomes increasingly difficult to hold onto a territory, so territoriality decreases and more males exhibit patrolling behavior.
In the speckled wood butterfly females are monandrous; they typically only mate once within their lifetime. On the other hand, males are polygynous and typically mate multiple times. In order to locate females, males employ one of two strategies: territorial defense and patrolling.
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