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Why are some insects so shiny and iridescent?

First here’s a post I answered on HOW insects form shiny or iridescent exoskeletons. It’s always good to know the answer to how because it can give context to the why. So why are some insects iridescent or shiny? 

This can be split into two main categories; communicative functions and non-communicative.  

COMMUNICATION

1. Mate Selection 

While insects tend to use chemical communication more as a means of attracting potential mates colouration can however play a significant role in mate selection in a number of ways. 

Honest signalling - in which the colouration reveals the quality of the individual as these colourations are costly to produce. Therefore only individuals that are high-quality are able to afford the cost of producing these signals (in this case the iridescence or shiny colouration)

Example: 

A study (Fitzstephens & Getty, 2000) found that male Calopteryx maculata (Black-winged damselfly) with higher fat stores had a much bluer iridescent colouration compared to males on low fat diets.

Amplifier traits - iridescent / shiny colours may be used to amplify the differences in the signals of quality (however no studies have directly focused if this is a function of iridescence)

Sensory drives - iridescent colouration was favoured due to being more effective in signalling in certain ecological environments 

Receiver Bias -  females (or in rare cases the males or hermaphrodites) as the receivers may have favoured certain iridescent colouration due to being better received by their sensory system therefore resulting in the evolution of this colouration

2. Species Recognition

Man species use iridescent / shiny colouration in order to identify members of their own species! As simple as that! 

Example: 

Colias eurytheme (orange sulfur butterfly) use UV iridescence to communication with conspecifics

3.Agonistic interactions

Iridescense / shiny colouration may be used in intrasexual encounters; mostly male-male agonistic encounters and can be used as a territorial signal.

4.flocking behaviour

Can help insects that school together facilitate their orientation or direction within their schools / flocks. This is seen in many aquatic species of fish and even squid. 

5.  Predation avoidance 

Iridescense / shiny colouration can be used to order to actually avoid predation! Although at first glance you’d assume this would make them more visible, this isn’t always the case. 

Mimicry / camouflage - insects can avoid predation by mimicking objects, leaves, raindrops, other harmful species or even by blending into the background 

Examples: 

A number of iridescent jumping spider species (Salticidae);  such as Brettus adonis in this study (Jackson & Hallas 1986) were found to mimic raindrops to avoid predation   

The iridescent green leaf beetles such as the  (Dogbane Leaf Beetle, Chrysochus auratus) similarity use their iridescence colouration to mimic dew on leaves.

Species like tiger beetles ( Cicindelinae) even use their iridescence to create an unsaturated appearance that allow them to blend into their envrionment (Schultz 1986, 2001)

Warning colouration - insects may use their colouration to communicate their toxicity or unpalatability serving as aposematic warning.

Example: 

Panamanian tortoise beetle (Charidotella egregia) that change from gold to red when disturbed by predators(Vigneron et al. 2007).

Startle displays - some species will use their iridescence colouration to create a flash that may startle potential predators long enough for them to escape due to the way in which the iridescence reflects light.

Example:

The tiger beeltes again! Some of them have bright colouration like below that they use as startle defenses against predators (Sargent 1990). 

NON-COMMUNICATION

1. Thermoregulation

There’s much debate over whether Iridescense / shiny colouration has any function in either heat absorption or dispersion. 

Some found evidence that the structures used to created iridescense / shiny colouration acted as heat collectors, like in the wings of butterflies ( Miaoulis & Heilman 1998). However other’s have found no evidence of thermoregulation in tiger beetles Schultz & Hadley (1987). 

2.Friction reduction

iridescense structures may reduce the friction in burrowing insects  

Example: Carabid Beetles (Brachininae) (Seago et al. 2009). 

These are just some of the proposed and studied functions of iridescense and shiny colouration in insects, there are more that aren’t as well studied or understood just yet! 

More reading: 

Doucet S, Meadows M 2009, ‘Iridescence: a functional perspective’, Journal of the Royal Society Interface, vol.6, no.2, pp.115–132 

Meadows M, Butler M, Morehouse N, Taylor L, Toomey M, McGraw K, Rutowski R 2009, ‘Iridescence: views from many angles’, Journal of the Royal Society Interface, vol.6, pp.203–211

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