Aposematism; the flashiest form of defence

Bright colours, like red, yellow, and orange, even in the human world are colours that indicate danger. For example, picture the red flashing don’t walk sign. You know to stop, and to proceed means danger. This doesn’t only happen in the human world; animals understand this too. Insects that display these colours say to predators, “don’t eat me I taste bad”.  

Like learning you need to stop at the red don’t walk sign as a child, the association of this colour with danger is often learnt. Predators, such as birds, learn to associate bright colours with a bad taste and possible toxicity. This is called “learned avoidance”. This leads them to avoid it in the future. Some insects have developed a very effective way to survive by warning predators of this. This display is called aposematism.

Although, you have to wonder why insects evolved to have a brightly coloured display in the first place. You would think that these bright colours would only attract predators as it makes them more noticeable. When insects are in groups with obvious movements, it is at no extra cost to the insect to display these colours. This means that it doesn’t negatively impact them. Other animals are more likely to notice these insects that have bright colours. However, they are less likely to attack them.

One example of an insect that has an aposematic display is the Hibiscus Harlequin Bug (Tectocoris diophthalmus). The adults are mainly bright orange. This colouration has found them especially well protected against birds. This insect also has an extra special aposematic display on top of their bright colours. They’re also iridescent (in other words, incredibly shiny). So, in a world where everyone else decided to blend into their surroundings, these insects decided to be extra fabulous. Studies have found that iridescence is also an effective way of warding off birds.

There is a lot of variation between individuals in the amount of colour and iridescence they display. Despite this variation, if it was between the Hibiscus Harlequin Bug and an insect that displayed no aposematism at all, it isn't a tough choice for the birds. Birds will still choose insects that have no aposematism display. So why the variation? Evolutionary advantages such as iridescence level can often be influenced by other factors. In the case of the Hibiscus Harlequin Bug, colour and iridescence are impacted by temperature and rainfall⁷. Other insects are influenced by humidity in the iridescent markings they display.

Despite aposematism making these insects much more noticeable, it has proven to be a very effective way to survive. So, while you’re outside, try and see if you can spot these beautiful insects (with those bright colours, it isn’t too hard). But remember to respect their warning signals. They’re trying to do you a favour!

Written by Elise Oakman from the School of Life and Environment Sciences.