Scientists have been baffled by the appearance of a rare type of beetle on a Hebridean island, hundreds of miles from their only other known habitat in Devon, south-west England.

The short-necked oil beetle, Meloe brevicollis, thought to have been extinct until a colony was found in Devon in 2007, has also been present on Coll since 2010.

Now a conservation charity is hoping a public survey of oil beetles in Scotland will find some answers to the mystery.

The beetle is one of three species of oil beetles in Scotland but Buglife project officer Dr Scott Shanks, said: "The one that is really rare is the short-necked oil beetle.

"It was thought to be extinct for about 40 or 50 years, but back in 2007 a tiny colony was found down in Devon, which caused great excitement.

"Then in 2010 a colony was found on the island of Coll in the Inner Hebrides. How they got here or whether they have always been there is still unknown."

He said one of the possible explanations was that oil beetles often climb on to the backs of solitary bees (not the social bees like the honey or bumble bees) when they land on a flower and then are carried around by the bee back to her nest where it snacks on the ball of pollen it has prepared.

"It is possibly one of the reasons the short-necked oil beetles got out to Coll: on the back of bees," Mr Shanks said. "So it is quite possible they are also in nearby islands such as Tiree and Mull as well as the mainland."

Mr Shanks said the survey could provide important new information, and not just about the short-necked oil beetle.

He said: "All oil beetles are facing declines due to loss of habitat and bees, on which they rely for part of their life cycle. We hope our new Scottish oil beetle guide will help the public identify these fantastic beetles and people can then submit their photos and records to the Buglife website."

The rugged oil beetle, Meloe rugosus, and the Mediterranean oil beetle, Meloe mediterraneus, have been found in England, not in Scotland.

According to Buglife, oil beetles are conspicuous, charismatic insects that are often encountered when out walking and enjoying the countryside.

Their habit of seeking out bare compacted earth in which to dig nest burrows means they are frequently seen on footpaths. The best time of year to look for them is March to June.