My childhood memories of Christmas holidays on the Cape south coast always recall the happy sound of the Cape Turtle Dove and the shrill song of cicadas in the midday heat. In the southern hemisphere, where Christmas falls at the height of summer, cicadas are known to us as Christmas Beetles or Sonbesies. They are indeed besies or bugs (Hemiptera family) and not beetles. So they should rightly be called ‘Christmas Bugs’.
Cicadas are found on every continent in the world, except for Antarctica, because cicadas don’t like the cold. There are about 150 species of cicada in South Africa.
When I started writing this article it occurred to me that I had not heard cicadas for some years. When I mentioned this fact to my son he was astounded. “What do you mean you haven’t heard them? They are splitting my ears right now.” I had not realised that my ageing ears could no longer hear their screech in the 0.5 to 25 kilohertz range.
The song of the cicada
Only male cicadas sing, and only those belonging to the group of cicadomorpha can produce sounds that are audible to humans. Cicada sounds have been recorded up to 120 decibels, which is close to the level of jet planes and can damage human hearing. Unlike similar insects, such as crickets, the male cicada does not use stridulation (the rubbing together of body parts) to make their loud sound.
Instead, they use a pair of tymbals, special structures on each side of the abdomen, just behind the last pair of legs and covered by a membrane called a tympanum. The tymbals are ribbed membranes, each having strong muscles attached to it. Contracting and relaxing these internal tymbal muscles causes the tymbals to rapidly vibrate and produce pulses of sound. In some cicada species, a pulse is produced as each rib buckles. The sound so produced is further amplified by the almost hollow abdomen which serves as a resonance chamber.
As mentioned before, only male cicadas sing and each species has its own song. They do it, of course, to attract a female of the right species – who responds to the song by flicking her wings.
Cicadas have an interesting life cycle
Cicadas are insects and the breeding cycle begins when adults mate after the males lure the females with their attractive cicada songs.
After mating, the female lands in a tree, and she uses her egg-laying organ (ovipositor) like a saw to cut a slit into the bark where she lays her eggs.
When the eggs hatch, the nymphs drop to the ground and they burrow down to depths of 2.5 metres into the soil. They then excavate chambers close to plant roots. There they feed on the juices from the tree roots by inserting their sucking mouthparts into the roots. They may spend up to 17 years and go through several ‘instars’ (developmental stages).
Cicadas have the amazing ability to synchronise their development so that they all reach the final instar stage at the same time. This is when they all tunnel to the surface at once, leading to a ‘plague’ of many thousands of cicadas. The instars crawl up plants where they shed their skins for the final time, morphing into adulthood.
The 17-year cycle occurs most notably in the USA. In 1987, when visiting the north-east USA, we saw branches breaking off trees under the weight of the huge numbers of cicadas that emerged that year.
This is the stage when the singing starts. The males die after mating and the females die after laying their eggs. So they don’t have much of a life other than the troglodytic one of the nymphs.
What are the biological advantages of a long instar phase and synchronicity?
There are some theories that explain advantages to the cicadas of ‘outsmarting’ their predators. Potential predators cannot rely on this food source year after year. One year they are there, and then they may be gone for anything up to 17 years.
An unreliable food source means that the predator numbers remain low. Because the cicadas all emerge in their thousands at the same time, it means that predators are ‘overwhelmed’ by the numbers as they can only eat so many at one time.
Other defence mechanisms
Cicadas have excellent camouflage. They look like bark, with disruptive patterns on their bodies that allow them to blend in with tree bark. This is why you will hear many cicadas singing, but it’s almost impossible to spot them. They have a capacity to ‘throw’ their sound, which further confuses us. Cicadas are excellent ventriloquists – they are never where you expect them to be.
Cicada wings do not reflect light, thereby preventing the typical insect cuticle shine. They are also very strong fliers and are able to fly at high speeds to escape their predators.
These facts about cicadas illustrate the evolutionary truth that any species exists only to continue to exist. Cicadas have evolved a complex life cycle which optimises the survival of every generation.
Wishing you many happy Christmas Bugs.
About the Author
Whale Coast Conservation passionately lives by its slogan “Caring for your environment”.
Its small staff and volunteers are dedicated to
- raising community and visitor awareness of the unique, biodiverse natural resources of the Cape Whale Coast region and
- to projects that convert awareness into practical actions that lead towards living sustainably.
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WCC increases general public awareness of sustainability through environmental education, citizen-science research projects, community projects and campaigns.
WCC communicates with its audience through exhibitions, signage, technology demonstrations, workshops, talks, film shows, newsletters and articles.
WCC places emphasis on educating future generations through its Youth Environment Programme (YEP). YEP is offered to 24 schools in its target area with a total of over 10,000 learners.
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WCC facilitates educator development programmes to improve the capacity of educators to offer informed environmental content in their lessons across all learning streams.