by Susan, Discovery Centre Information Officer

Jewel beetles (Buprestidae) and Christmas beetles (Scarabaidae) are common names given to particular groups of beetles due to their spectacular iridescent or metallic colouring. The gold, green, blue or purple colour shifts as the insect moves delighting us (and presumably any potential mate) with their beauty. But what causes these shifting colours?  

Colour in nature is produced using many mechanisms including pigments, bioluminescence and certain structures. Pigments are chemical colours such as melanin (browns) and carotenoids (yellows and oranges) that combine to form colours for example in many butterfly wings. Structural colour, on the other hand is produced when fine grooves or a series of fine layers on scales, hairs or on the exoskeleton (outer hard casing) diffract light causing us to see a particular ‘colour’.

Beetles are not the only group to use this mechanism. Bee flies, for example have scales that are black when viewed from one direction, but appear brilliant metallic maroon, green, or blue if viewed from a different angle. Pigments differ from structural colours as the colour stays the same when viewed from different angles and may change as an insect ages or fade when the insect dies and is exposed to light. This means that butterflies put out on display will lose many of their pigmental colours. Structural colour will not fade over time, so those beautiful beetles, bee flies and butterflies will retain their metallic colours for many years after the insect itself is no longer alive! Hence their popularity in displays or, as in the case of Jewel beetles as jewellery in Victorian times.