They thought this butterfly had flown away for good …

Natural History Museum in Aarhus calls it a “sensation”

A caterpillar of the common yellow swallowtail butterfly (papilio machaon), which has not been seen in Denmark for 40 years, has been found in the forest of Hodsager Plantage in west Jutland.

The caterpillar was found by Carl Johnsen and Ole Olesen, two residents of the nearby town of Aulum, who later posted photos of the larva online.

“It was an incredibly beautiful caterpillar, so I took several pictures of it,” Johnsen told DR.

Morten DD Hansen, an expert from the Natural History Museum in Aarhus, has called the find “a sensation”.

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“We have been waiting for this for many years,” Hansen told DR.

“So let’s just hope they turn into butterflies!”

Earlier this summer, eggs of the common yellow swallowtail were found at Gentofte Lake near Copenhagen.

The new discovery suggests the species may have found its way back to Denmark after many years of absence.

The butterfly is widespread across Europe, but protected in many countries.