Today we are featuring the beautiful metallic beetle Charidotella sexpunctata, also known as the Golden tortoise beetle. These North American beetles are small, measuring approximately 5 – 7 mm in length and are instantly recognizable due to the expanded margins of their prothorax and elytra. The outermost margins of these expansions are almost completely transparent and form the “tortoise shell” appearance that gives this beetle its name.
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 or otherwise comfortable, the shell appears a brilliant and almost metallic golden color; often with hints of green or blue. When disturbed, however, the color will to shift to a red-with-black-spots coloration. A similar color shift also occurs when the beetles are copulating, but instead of red-with-black-spots they assume a shimmering crimson tone.
Such reversible color change is rare in insects and a possible proposed mechanism for C. sexpunctata’s color shifts wasn’t available until 2007 when researchers from Belgium were able to use 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 these layers is a primarily red pigment layer that makes up the beetle’s inherent coloration.
When filled with liquid, the upper layers of the “shell” are able to refract a substantial portion of the incident light (almost like a mirror) resulting is the spectacular metallic and occasionally almost iridescent coloration noted when the beetle is at rest. If the beetle is disturbed, it drains these layers and consequently 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 necessary to create the reflection, only the underlying pigmentary colors remain. Though C. sexpunctata specifically has not yet been studied to the level of it’s Panamanian cousin, it is likely that it employs a similar fluid-mediated switchable mirror in it’s own color shifts.
But why shift color in the first place? Based on behavioral research conducted in 1979 by Edward M. Burrows, it appears 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 bears a decent resemblance to a Ladybird beetle, it’s possible that C. sexpunctata has evolved an appearance that mimics the 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, there is much more work 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:
- Barrows, Edward M. “Life cycles, mating, and color change in tortoise beetles (Coleoptera: Chrysomelidae: Cassidinae).” The Coleopterists’ Bulletin (1979): 9-16.
- Golden tortoise beetles – Department of Entomology and Nematology, University of Florida.
- Vigneron, Jean Pol, et al. “Switchable reflector in the Panamanian tortoise beetle Charidotella egregia (Chrysomelidae: Cassidinae).” Physical Review E76.3 (2007): 031907.