Saturday, 25 May 2013

On the wing: Speckled Wood

P.aegeria tircis (N.Europe)
Now flying in the woods opposite the zoo, the Speckled Wood Pararge aegeria is one of our commonest butterflies, and is familiar to anyone who has gone for a woodland walk in most parts of the country. It has an amazingly long flight season, and can be seen from now until September, even though an individual butterfly probably only lives for a week at most.

Keen observers will notice that there is a good deal of variation in appearance over the course of a season, and this butterfly is surprisingly variable in many ways. Generally, early emerging individuals tend to be shades of cream and darker brown, with later individuals maturing in warmer weather tending to have more yellow and orange shades. This variation becomes even more extreme in the form from southern Europe, which tends to be much brighter than the northern European form.
The variation also extends to the life cycle. The eggs are laid on the blades of several species of grass (even bamboo in ornamental gardens), with the site chosen depending on the season – warm spots in the cooler months, more shaded locations in the heat of summer. The caterpillar matures in a month or so, and the pupa takes about 10 days to hatch if it is going to emerge the same year. This is not always the case, as late maturing larvae may overwinter as a pupa. They may also overwinter as larvae, burying themselves deep in a tussock of grass, and this variation explains the long initial emergence period. Depending on when the adults emerge, there may be two or three generations each year before cooler weather and shortening day length causes them to switch to hibernation mode.
One of the most distinctive features of the Speckled Wood is the territorial behaviour of the males. In cooler periods of the year especially, they tend to perch in the sunshine looking out for visiting females and rival males. If a rival is spotted they engage in fierce dogfights, with the intruder usually being driven off. In warmer weather some males become patrollers instead, flying around much larger territories in search of females. The two behaviour patterns are in part genetically determined, and patrolling males tend to have four spots on the upper side of the hind wing where perching males have three. Perching males also have a larger thorax to house larger wing muscles that give better acceleration from a standing start.

(images from wikipedia)

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