Life Science SERIES: Invertebrate of the Week

Invertebrate of the Week #8 – Golden Tortoise Beetle (Charidotella sexpunctata)

Close up photograph a golden tortoise beetle Charidotella sexpunctata preparing to take off from a leaf and fly.
Golden tortoise beetle preparing to fly.
Photo by Weng Keong Liew

Today we are featuring the beautiful metallic beetle Charidotella sexpunctata, also known as the Golden tortoise beetle. These North American beetles are small. They measure just 5 – 7 mm from end to end but are instantly identifiable due to the conspicuously expanded margins of their prothorax and elytra.  

The outermost margins of those expansions are almost completely transparent and form the “tortoiseshell” appearance that gives this beetle its name.

Close up image of the dorsal aspect of a tortoise beetle Charidotella sexpunctata in the resting state, including a clear view of the transparent elytra.
Golden tortoise beetle in resting state.
Photo by Bob Cammarata

Changing Colors to Fit the Mood

Of course, these beetles’ most obvious and eye-catching trait is the metallic gold appearance of the remainder of the “shell”. When the beetle is at rest and relaxed, the shell becomes a brilliant metallic golden color; often with hints of green or blue.

When disturbed, however, the color will shift to a bright red-with-black-spots pattern. A similar color shift also occurs when the beetles are copulating, but instead of red-with-black spots, they become a shimmering crimson tone.

Dorsal view of a golden tortoise beetle Charidotella sexpunctata with red coloration with black spots
Golden tortoise beetle in red coloration with black spots.
Author: http://goo.gl/2tbHhg

Such reversible color change is rare in insects and a possible proposed mechanism for C. sexpunctata’s color shifts wasn’t available until 2007. Researchers from Belgium were using electron microscopes to investigate the shell structure of the closely related Panamanian tortoise beetle, Charidotella egregia. They found that the beetles’ shells are composed of a “stack of thin layers with thicknesses that increase with depth to produce a broadband reflector.”  Underneath those layers is a primarily red pigment layer – the beetle’s inherent coloration.

Golden tortoise beetle Charidotella sexpunctata specimen photograph dorsal view with high resolution details of the elytra and coloration.
Golden tortoise beetle specimen (Charidotella sexpunctata).
Photo by the USGS

Bending Light with Fluid

When filled with liquid, the upper layers of the shell refract a substantial portion of the incident light (almost like a mirror). This produces the spectacular metallic occasionally iridescent coloration noted when the beetle is at rest.

If the beetle is disturbed, it drains these layers and disrupts the mirror effect to expose the red and black undertones below. The researchers called this phenomenon hygrochrome and likened the beetle’s shell to a switchable mirror.

This theory also explains why dry and/or dead specimens lose their metallic, golden appearance. Without the moisture required to create the reflection, only the underlying pigmentary colors remain.

Though C. sexpunctata specifically has not yet been studied to the level of its Panamanian cousin, it likely employs a similar fluid-mediated switchable mirror in its own color shifts.

A ladybird beetle on a leaf
The coloration of a disturbed golden torquise beetle may be an evolutionary attempt at mimicking the coloration of an unpalatable Ladybird beetle (Coccinellidae)
Author: http://goo.gl/lGyWBT

Aposematism Without All the Work

But why shift color in the first place? Research conducted in 1979 by Edward M. Burrows suggests that the color shifts may serve at least two distinct functions. The first proposed function involves sexual signaling. Burrows observed that young beetles appear to be incapable of producing the golden coloration found in adults and thus a shift to gold may serve as an indicator that an individual has completed “beetle puberty” and is ready for…adult activities.

The second proposed function concerns the shift from gold to red-with-black spots that occurs when the beetle is disturbed. Burrows noted that certain insectivorous birds find golden tortoise beetles very appetizing but avoid eating the similarly-shaped Ladybird beetles (Family: Coccinellidae) which they find unpalatable.

Since a red-with-black-spots golden tortoise beetle resembles a Ladybird beetle, it’s possible that C. sexpunctata evolved an appearance that mimics that of the unpalatable Ladybirds. Such behavior is known as Batesian mimicry and occurs when a harmless or otherwise palatable species has evolved to imitate the warning signals of a harmful or otherwise unpalatable species as a means of defense against predation.

Obviously, much more research needs to be done before we fully understand all the intricacies of this remarkable insect but in the meantime, I’m certain that its spectacular coloration will continue to enthrall all degrees of naturalist for generations to come.

References and Further Reading

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