Tortoise beetles are quaint insects that come in various colours, from metallic gold and green, to matt red or brown, some with intricate squiggly designs or marked spots. Their common name comes from their dome-shaped top shell, which is similar in shape to the shell of tortoises. This hard shell is called a carapace and is the top exoskeleton of the insect, functioning primarily as a protective covering against mechanical pressure and dehydration.
Anatomically, it is made of hardened forewings, called elytra (singular elytron) and the protonum, which is a plate-like cover that protects the thorax and head. Elytra cover the membranous hindwings used in flight. The carapace is made of layers of a biopolymer called chitin, embedded with proteins and minerals for strength. The structural properties between the inner and outer layers of the carapace differ, endowing them with different functions, similar to the layers of human skin. Interestingly, the composition of all the layers of the carapace together may contribute to the colours and shine of the tortoise beetle.
In some species, the carapace plays a unique role in predator evasion. “These beetles have the most interesting ability to change the colour and reflectivity of the carapace, a phenomenon called ‘optical defence’,” says Priyanka Ghosh, senior research fellow at the Zoological Survey of India, Kolkata who has studied them in West Bengal. This type of defence in adults may confuse predators as their shiny green target appears to dissolve into the surroundings. A few species also appear to change colour in response to temperature or humidity changes.
Tortoise beetles (subfamily: Cassidinae) belong to the leaf beetle family (Chrysomelidae), which has over 3,000 species. Tortoise beetles eat leaves, favouring sweet potato (Ipomeoa batatas), morning glory (Ipomeoa triloba), coffee (Coffea sp.), and curry leaves (Bergera koenigii) and are therefore often regarded as pests. However, they rarely have a large enough infestation to cause serious damage to crop yield. In the larval stage, these beetles are more gregarious than as adults. Newly hatched larvae scrape and eat the top of the leaves, leaving behind a lace-like translucent filigree.
In a behaviour unique to Cassidinae, the larvae of many species craft a “shield” as they do not yet have the protection of the adult carapace. This shield is carried on its back, supported by a fork-like structure closer to its rear end. It folds over the front body somewhat like a scorpion’s tail but with a protective function.
These small beetles teach us the evolutionary use of combining form, colours, behaviours and unique craftsmanship in escaping predators. They are important herbivores that maintain the ecological equilibrium of plant species and may also be pollinators.
So, next time you see irregular holes or filigree in leaves of the neighbourhood’s morning glory, turn over a new leaf and look underneath at a wonderful tortoise beetle you share the world with.
During field collections, Priyanka Ghosh of ZSI observed that the (1) green, patterned Cassida circumdata, (2) green, spotted Chridopsis bipunctata, and Cassida viridi (not shown) change colours from green to brown as a defence mechanism against predators. The mechanism of colour change is complex and different from that of chameleons or marine animals. Detailed studies have been conducted on their cousin, native to the Americas, the golden tortoise beetle (Charidotella egregia). When disturbed, this beetle changes from a metallic golden to a matt red in 90 seconds. The colour change is associated with changes in moisture content in the elytra. Moisture gives the golden metallic hue, while dryness (and death) is associated with the matte red. The study required a nano-scale investigation done by Jean Pol Vigneron and other scientists. In their paper published in Physical Review E, in 2007, they stated that when disturbed, the beetle loses its mirror-like reflection from the outer layers of the carapace; thereby, the deeper inner layer containing red pigmentation becomes visible with a matte effect. It is proposed that the beetles have a way of actively moving fluid or water out of their elytra’s outer layers for this transformation process. Photos: Jithesh Pai
(1) The curry leaf beetle (Silana farinosa) on its host plant. It has a waxy white deposit on its carapace instead of a colourful metallic one. The colours or deposits on the carapace serve as camouflage to prevent easy detection depending on the host plant or habitat. (2) The face, antennae and underside of the beetle Aspidimorpha miliaris. The extended carapace spreads outward and beyond the body of the beetle. This allows an additional mode of defence as the beetle can flatten its body, fold its legs, and clamp down on leaves using this extension, making it difficult for ants or other predators to carry it away forcefully. Photos: Jithesh Pai (1), Hayath Mohammed (2)
(1,2) Larvae with self-made defensive shields on their backs and spiny edges around their bodies. The larvae of tortoise beetles make exemplary use of sustainable building materials to construct a “shield”. The framework of the shield is made of exuviae: the skin they shed as they grow. The shed skin is pushed towards the back and sticks to a pair of special projections that look like a fork. At each of the five larval growth stages (called instars), the skin peels off and is added to the shield, almost like links in a chain, as seen in (2). This is not a round shield. The framework of the shield is inlaid with their own poop using the deft telescopic anus in most of the ~2,600 species of tortoise beetles, “The larva’s telescopic anus is the only tool used to build and repair the shield, not mouthparts or legs”, write Dr Caroline S Chaboo and others in their article published in the journal Zookeys (2023). It is believed that the noxious fumes from the faeces deter predators. In a soldier-like manner, the shield can be moved or wielded around by the fork to distract or ward off predators like jumping spiders. The shield may also protect them from extreme temperatures or desiccation (drying).
Evolution has allowed the larvae some more protective measures. As seen in pictures 1&2, there are spines or spikes present at the edges of the larval body. This form of optical illusion (that the larvae use passively) confuses predators. Predators perceive their prey based on the clear outline and shadows; even relatively flat things cast shadows, making them easy to find. The spines or spikes at the edges of these bodies work to disrupt a sharp shadow outline, making them almost imperceptible to predators like wasps. This adaptation is also found in the pupae. Photos: Hayath Mohammed
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