Nature on our doorsteps: Shiny beetles

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into the natural heritage around us and the beautiful biodiversity of the plants and creatures.

The many wonderful colours in nature are a result of the wide range of pigments that occur in leaves and flowers, in wood, in the feathers of birds, in insects, and in animal hair and fur.

The metallic green, blue, gold, red, and black colours of some insects, however, are not always a result of pigments.

Shiny colours in insects are often due to what is termed ‘structural colouration’, meaning the colour is a result of how the insect’s outer layer of hardened skin is structured.

Insects are invertebrates.

They have no internal skeleton like animals and birds.

Instead, their outer skin is hardened, and is referred to as an exoskeleton, or an ‘outside skeleton’.

The hardened exoskeleton around a beetle’s body is called a carapace, and these can sometimes appear very smooth and shiny with metallic, vibrant colours.

When viewed under a microscope, a beetle’s carapace is seen to be covered in tiny ridges or layers.

As light lands on these ridges they act like many different lenses.

The light is scattered back outwards in such a way that causes some light wavelengths to be cancelled out while others are made more vibrantly visible.

This is the same process that causes the rainbow effect when light lands on the pitted surface of a DVD or a CD.

The carapaces of some species of beetle scatter light in a consistent way, so that they always appear a particular metallic colour.

Other species use a different method of scattering light called iridescence, which causes their colour to appear to change, or briefly flash, as they move.

It is suggested that the metallic shine might help beetles find a mate in the darker habitats in which some of them tend to live, while flashy iridescence may help scare away potential predators.

Colours created by pigment compounds take more energy to make than structural colours.

As insects shed their old skins in order to grow, there is therefore much less energy wasted in the use of structural colours.