Fascinating Insects: The Cicada
I think it is safe to say that summer is officially here by just listening to the bush. Most of the migrant species of birds have returned, and the bush is starting to get lush and green. We had our first rains a couple of weeks ago, and all forms of life, most notably insect life, have exploded since then. One characteristic sound that is back is the constant background noise of the cicadas.
Cicadas are fascinating insects belonging to the order Hemiptera, commonly known as true bugs. They are part of the suborder Auchenorrhyncha, which includes other small, jumping insects like leafhoppers and froghoppers.
Most cicada species are referred to as annual cicadas because they emerge yearly in varying numbers. However, some North American species from the genus Magicicada are known as periodical cicadas. These unique species emerge in synchronized masses once every 13 or 17 years, depending on their life cycle. A common cicada we find here in the Greater Kruger area is the Orange-wing (Platypleura haglundi).
Cicadas are indeed notable for their large size and the loud courtship songs produced by males. These insects are characterized by several distinct anatomical features:
- Tarsi with Three Segments: The tarsi, or the distal segments of their legs, are composed of three joints. This feature is common among insects in the Auchenorrhyncha suborder, which includes cicadas, leafhoppers, and planthoppers.
- Small Antennae with Conical Bases: Cicadas have relatively short antennae that emerge from conical bases. These antennae consist of three to six segments, with the final segment ending in a fine hair-like structure called a seta, which aids in sensory perception.
- Prominent Tymbals in Males: A distinctive feature of male cicadas is their ability to produce sound using specialized structures called tymbals. These are located on the sides of the first abdominal segment. Male cicadas are renowned for their ability to produce loud, rhythmic sounds, which they use primarily for courtship. These sounds are generated by a specialized structure called the tymbal, located on the sides of the first abdominal segment. Each tymbal consists of a pair of resilient, membrane-like plates made of a protein called resilin, which allows for rapid and repeated deformation. The sound production process involves:
- Muscle Contraction: Strong muscles attached to the tymbals buckle these membranes inward.
- Elastic Rebound: Upon muscle relaxation, the resilin-rich tymbals snap back to their original shape, creating a clicking sound.
- Amplification: The hollow abdomen of the male cicada acts as a resonating chamber, enhancing the volume of these sounds.
This mechanism allows male cicadas to produce their characteristic songs, which can reach remarkable volumes of up to 120 decibels in some species. The songs vary among species in rhythm and pitch, serving as a means of attracting mates and distinguishing themselves from other species.
In cicadas, reproductive and life cycle behaviours vary by species. Males typically adopt one of two strategies to attract mates. Stationary calling in many species where males remain in one location and produce loud calls to attract females and mobile calling in other species the males move from place to place while emitting quieter calls, actively searching for potential mates.
After mating, females use their ovipositors to cut slits into the bark of twigs and deposit their eggs. Both sexes die shortly after reproduction, with an adult lifespan of less than two months. While they can consume small amounts of plant sap for sustenance, feeding is minimal during this stage of their life.
Life Cycle
- Egg Stage: Eggs hatch into nymphs, which drop to the ground and burrow.
- Nymph Stage: Cicadas spend the majority of their lives underground, feeding on xylem sap from plant roots. Nymphs are equipped with strong forelimbs for digging and often coat their burrows and bodies in fluid secretions. In wet environments, some species construct mud towers above their burrows to improve air circulation.
- Final Instar: In their final nymphal stage, they construct an exit tunnel and emerge to moult (shed their exoskeleton) on a nearby plant. This marks their transformation into winged adults.
The abandoned exoskeletons are often found clinging to tree bark, a familiar sight during cicada emergence events.
Most cicadas are diurnal and rely on a variety of strategies to evade predators. Large cicadas can fly rapidly to escape if disturbed. Many are extremely well camouflaged to evade predators such as birds that hunt by sight. Being coloured like tree bark and disruptively patterned to break up their outlines, they are difficult to discern; their partly transparent wings are held over the body and pressed close to the substrate. Some cicada species play dead when threatened.
Cicadas are sap-feeding insects that typically do not harm humans or animals directly. While they lack biting or stinging mechanisms, they may occasionally mistake a human arm for a plant limb and attempt to feed, which can cause mild discomfort. Their feeding habits generally involve extracting sap from plants using specialized mouthparts called stylets.
Human Interaction
- Hearing Damage: Male cicadas are known for their loud calls, which in some cases can reach levels that may damage human hearing, especially when heard at close range.
- Non-Aggressive: Cicadas do not bite, sting, or inject venom, and their interactions with humans are generally harmless.
Impact on Agriculture and Plants
- Egg-Laying Damage: Female cicadas create slits in tree branches to lay their eggs, which can cause structural damage:
- Small trees may experience wilting or branch dieback.
- Larger trees typically recover, but they may lose smaller branches.
- Feeding by Nymphs: While the feeding activities of nymphs are generally minor, in the year preceding a large cicada emergence, the feeding pressure from mature nymphs can reduce plant growth.
- Crop Damage:
- Some cicada species may adapt to feeding on agricultural crops like sugarcane, affecting yields.
- In rare instances, females have laid eggs in economically significant plants such as date palms, grapevines, citrus trees, asparagus, and cotton, causing localized damage.
Cicadas can harm ornamental shrubs and young trees primarily by leaving scars on branches from egg-laying activities. The damage is usually cosmetic, but in some cases, it can result in the death of affected branches.
Although cicadas are not major agricultural pests, their periodic mass emergences can create localised challenges for farmers and gardeners. Most plants, however, are resilient to cicada activity and recover after an outbreak subsides.
