A tortoise among the leaves
Geetha Iyer
A few months ago, I had introduced Ipomoeas to you. If you found the opportunity to observe or study them, it’s quite likely that you may have noticed a pretty colourful beetle scuttling amongst them. Ipomoeas are favourite food plants for insects and so host several of them. Quite a diverse group of insects may be observed on these plants. One to be surely found is a beetle fondly called, ‘the tortoise beetle or tortoise shell beetle’.
Why are they called tortoise shell beetle? If you can observe them, then this question is easily answered. All insects have two pairs of wings. In beetles, the outer pair of wings or the forewings, called elytra, are not used for flight. They are hard and protect the inner pair of hindwings as well as the body beneath the wings. In tortoise shell beetle, this function is even more comprehensively carried out. The elytra in these beetles are expanded to cover the entire body. The pronotum (the first segment of the thorax) extends similarly over the head. Together these two structures form a protective shield, like a hood for the beetle. When the beetle perceives danger, it quickly withdraws its head, antennae and legs beneath the hood just like a tortoise withdrawing its body under its shell, hence earning the common name.
The expanded covering clamps down so firmly that hunters like ants, which grab their insect prey by the legs will not be able to penetrate the hood. The strong and firm elytra may deceptively appear to be one continuous transparent cover; but they are not. When it starts to fly, what appeared till then as a continuous shield suddenly splits in the middle, revealing the two halves of the wings. The forewings move outwards and upwards, to allow the inner hindwings to spread and fly. Because the elytra are somewhat transparent, the patterns and colours on the body are visible, making it one of the most attractive kinds among beetles.
Some of these beetles have spots on their body, so are often mistaken for ladybirds. The two beetles belong to separate families and have different lifestyles. Ladybirds can be carnivorous, whereas these beetles are vegetarians. Tortoise shell beetles belong to the Family Chrysomelidae – a very large and diverse family of leaf beetles. Since it’s a leaf beetle, it is not surprising that you find it on the leaves of the Ipomoea, its favourite food plant. You can see them on leaves of plants that belong to the Family Convolvulaceae. There is a pattern to their feeding. If you find holes on the leaves of the railway creeper plant or the morning glory, it could be mostly the work of these beetles. This is not to say that there may not be other leaf beetles or the leaf-cutter bee at work; but generally one is sure to find a tortoise shell beetle or two on plants whose leaves appear perforated by small circular or irregular holes.
The beetles feed on the surface of the leaves and are hence easy prey. They have therefore devised ways to escape predation. The near transparent shield is an important adaptation. Another is to quickly scamper away to the underside of the leaf, remain closeted within the hood to reappear only when they perceive no danger. The colours – visible through the wings – also serve to ward off predators.
Larvae of tortoise beetles
The larvae are flattened and oblong in shape with spiny projections adorning the edges of their body. The food plant for their larvae is the same as that of the adults. Like the adult beetles, the larvae too must save themselves from predators. The adaptations seen in the larvae are even more intriguing. There are several tricks adopted for survival. The simplest is the colour. In some species such as Cassida, the larvae are green in colour camouflaging well with the leaves. The larvae in all species have spiky projections all around their body. These spines may make eating uncomfortable for the predator, but that is not the primary purpose, say entomologists. The spikes around the body create uneven shadows on leaves that confuse predators. Predators such as birds will take a few seconds more to focus and find the location of the larvae and not just swoop down on seeing them; this gives the larvae that much more time to move away to safety.
But the most intriguing part is the parasol they carry. Parasol might bring to your mind an image of girls with colourful umbrellas. But the parasol is nothing even remotely fanciful in these insects. Their parasol is a faecal brolly made from their shit and moulted skin. The posterior end of the larva has two spines on which are deposited their faecal remains or their moults, each over the other. The insignificant spike now changes to a small but bulky mass held over the spine. When disturbed, this spiny part with the accumulated remains is curved over to cover the body. The larvae thus become camouflaged to appear like an untidy mass. The beetle larva carries its moulted remains and faecal matter on its body as it goes about feeding or resting. When required they are deposited one over the other to construct the protective parasol.
Golden tortoise shell beetle
The tortoise beetles come in a variety of colours and the golden coloured beetle is also seen in India. These were among the first insects discovered that could change their body colour during mating. They also change colour when disturbed, going from golden to golden orange! This is an optical phenomenon. Researchers in Belgium have found that the beetle is able to fill or drain a liquid made of the red pigment present on the surface of its body. These events happen at the molecular level. The liquid when present reflects the light falling on it to produce the golden colour. When there is no liquid then this optical phenomena is absent and the beetle has an orangish hue.
The tortoise shell beetles are not serious pests except for select plants such as the sweet potato. For this reason it’s not a widely studied insect.
For the middle school class: Project Tortoise shell beetle
Habitat and adaptation are themes that are studied by students in classes 6-8. Tortoise shell beetles can be reared from the larvae in the lab. The observations will help learn concepts related to habitat and adaptation in a hands-on manner. The project can also be used for CCA or formative assessments.
You will need: A suitable container – a box/a small aquarium type glass tank; leaves of Ipomoea, Morning Glory or brinjal/tomato plants. Ipomoeas are weeds that can be easily collected in any city, not to mention rural areas. In cities, look for them near railway stations, or water bodies. A small plastic jar/glass bottle to collect the beetle and larvae.
Collect larvae of the green tortoise beetle Cassidacircumdata from the Ipomoea plant. Any tortoise shell beetle larvae will do. The image here will help you identify them.
Try to collect larvae that have just hatched from eggs or have moulted once. They will be small and green in colour. As the larvae eat, grow, and moult they change to a creamish brown colour. Larvae of other species will have the colour given in the image here. The image shows a well-grown larva that has moulted three times. This stage too is ok to collect. Collect carefully, preferably with the leaf on which you found it.
Set up the larvae inside the container with the leaves. In the corner of the container leave a small plastic bowl with some water. Make sure that the container is closed from the top with a cardboard in which some holes have been pierced for flow of air.
Observe and keep notes
The larval stage may last for about 20 days, but it will be fun to check out this data for yourself.
Watch how the larvae eat, moult, make their parasol and get ready to turn into pupae.
The larvae will stop eating and become sluggish. This is a sign that pupation will soon begin.
Make sure there are some fresh leaves because the pupae will attach themselves to the leaves by means of a fine thread. Watch the process of pupation, which can be quite interesting.
A pupa will take about a week to eight days to emerge as adult beetle.
If you manage to get the adults then continue with the activity to see the adult behaviour and also whether it would lay eggs to start another generation in the box or else release the beetle.
Recorded longevity for the adult beetles is about a month. Can be quite an exciting project.
The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.