Читать книгу Extreme Insects - Richard Jones - Страница 17

NAME	map butterfly Araschnia levana
LOCATION	widespread across mainland Europe
ATTRIBUTE	alternating generations of completely different-looking butterflies

The European map butterfly, Araschnia levana, gets its name from the pretty patterns that mark the undersides of its wings. The mottled browns and oranges of its background are criss-crossed with bright white lines reminiscent of the radiating compass marks superimposed on old maps and nautical charts. However, it is the patterns of the upper sides that are most remarkable.

Spring butterflies, emerging from chrysalises that have remained dormant through winter, are bright orange above, marked with a series of black spots and blotches. Their eggs produce caterpillars that feed quickly on their nettle host-plants, and the summer generation of butterflies that emerges a few weeks later has a completely different colour pattern – jet black, with a strong white flash down each wing (shown right). So different are these colour forms that they were long thought to be two distinct species.

This extreme dimorphism (meaning ‘two forms’) has attracted a lot of research from entomologists, and the factors that decide which colour pattern will be produced are now well understood. The final adult morph is decided by the effects of day length and temperature on the feeding caterpillar. Short days and cold, enough to induce winter torpor, produce the spring orange form levana while long hot days produce the black and white summer form prorsa. Experiments have shown that caterpillars from either generation can be raised under artificially altered temperature and daylight regimes to produce the ‘wrong’ adults.

It is still not known why the map butterfly shows such stark changes between its two generations. The scene is further confused by the fact that more northerly and montane populations have only one generation (form levana) each year, while in the south there is a partial third generation with intermediate levana/prorsa characters.

As well as different colour patterns, the summer form prorsa has larger and less pointed wings, a heavier (presumably more muscular) thorax and relatively smaller abdomen. These characters fit the idea that the summer form is better at migrating to colonise new regions (the spring form is noticeably more sedentary), but it still does not explain why one butterfly species should look like two completely different creatures.