Antenna 50 (1) Highlight – Featured Insect: Charidotella sexpunctata (Fabricius, 1781) – American Golden Tortoise Beetle
Article by Talay Namintraporn, Natural History Museum, London
At first glance, Charidotella sexpunctata looks less like an insect and more like a piece of jewellery due to its metallic golden colouration. This beetle belongs to the family Chrysomelidae and subfamily Cassidinae, which are known as the tortoise beetles. This name derives from their tortoise-like body shape which they utilise as a defence mechanism.
A tortoise beetle is broadly characterised by enlarged elytra and pronotum which cover their head and legs. The specialised pads on their feet allow the beetle to hold firmly onto a leaf, making it challenging for predators to remove an individual from their host plant (Eisner & Aneshansley, 2000).
The brilliant metallic colour of C. sexpunctata is not produced by pigment, but by fluid-filled microscopic ‘nanotube’ structures in its cuticle that manipulate light, an example of structural colouration (Biró & Vigneron, 2010). When the beetles dry out, they lose their golden colour.
Experiments to maintain their colour in museum collections have involved placing specimens in tubes of oil to prevent desiccation. The Latin name of the species C. sexpunctata means ‘six-spotted’, referring to the six spots on the elytra which become more visible when the beetle loses its golden sheen.
Image: A dried specimen of Charidotella sexpunctata. Credit: Talay Naminatraporn / Natural History Museum, London.
Charidotella sexpunctata is a widely distributed American species, however, there are several beetle species which are known as ‘golden tortoise beetles’ due to their similar colouration. These include Aspidomorpha sanctaecrucis and other closely related species which occur across Asia (Borowiec & Świętojańska, 2008–2024).
Female C. sexpunctata lay their eggs on the underside of leaves of Convolvulaceae host plants, which include sweet potato and bindweed. Unlike some other tortoise beetles, this species does not exhibit parental care; once laid, the eggs are left to develop on their own. The larvae feed on the same host plants as adults and are often gregarious, feeding in groups.
They are also known for exhibiting a unique defence mechanism: the ‘faecal shield’. Larvae accumulate their own faeces and shed skins on a forklike structure at the rear of their body and hold it over themselves like an umbrella. This shield deters predators such as ants and parasitic wasps (Barclay & Bouchard, 2023). Adult C. sexpunctata are about 5–7mm long and are characterised by their metallic gold appearance. When disturbed or stressed the beetle’s colour changes from shiny gold to a red or brown colour.
The mechanism for colour change in C. sexpunctata has not yet been studied. However, in the closely related species, Charidotella egregia (also known as the ‘Golden Tortoise Beetle’), the mechanism for colour change is a physical ‘expulsion’ of the reflective fluid, which exposes the red pigment beneath (Vigneron et al., 2007). It is also thought that the colouration and smooth reflective surface of the elytra may mimic water droplets on leaves, helping them blend into their environment (Barclay & Bouchard, 2023).
References
- Barclay, M.V.L. & Bouchard, P. (2023) Beetles of the world: A natural history. Princeton University Press, Princeton, USA.
- Biró, L.P. & Vigneron, J.P. (2010) Laser & Photonics Reviews 5, 27–51.
- Borowiec, L. & Świętojańska, J. (2008–2024): Cassidinae of the world – an interactive manual (Coleoptera: Chrysomelidae). https://www.cassidae.uni.wroc.pl/katalog%20internetowy/index.htm
- Eisner, T. & Aneshansley, D.J. (2000) Proceedings of the National Academy of Sciences 97, 6568–6573.
- Vigneron, J.P. et al. (2007) Physical Review E 76, 031907.
Featured Insect Information:
- Scientific name: Charidotella sexpunctata (Fabricius, 1781)
- Order: Coleoptera
- Family: Chrysomelidae
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