Summertime serenades
Summer in the Lowveld is in full swing and February, while universally known as the month of love, is also a time in the Kruger National Park of gloriously high seasonal temperatures but with humidity levels to match. This warmth coupled with more than decent rain showers of late has left the bush in tip-top condition and the Kruger’s browsers and grazers alike will enjoy the abundance of fodder for months to come.
These conditions are also optimal for insect activity and visitors to the Park over the summer will no doubt have seen many of our six-legged friends while on their travels. Just think of last month’s blog, when I described the emergence of the mopani worm and subsequent emperor moth after metamorphosis. Or the flights of alate termites after rains. But there would also have been much aural contact with insects and it’s here, dear readers, that I’ll be spending my time on this month’s blog. Insects produce sound in almost every conceivable way. Much like mammals and birds do, insects use the energy-efficient method of sound to communicate and since all insects lack lungs and vocal chords, they have adapted various body parts to make this possible. The beauty of sound is that it can be transmitted in innumerable tones and rhythms, with varying pitch and volume to create incalculable combinations of creep-crawly crooning!
Most sound produced by insects serves the purpose of attracting a mate. Think of the male cricket chirping incessantly all night long. We know it’s the male as the female cricket can make only very faint sounds, and then only when mating. The sound that crickets (and other insects) make is called stridulation which is defined as noise made by rubbing body parts together. In the case of crickets, the front edges of the forewings are rubbed against each other with the wings slightly raised to create a parabolic reflector. This serves to amplify and direct the sound making it near impossible to locate the cricket as it turns while chirping.
Other cricket varieties have developed ingenious methods to magnify their sound. Tree crickets cut a hole in a leaf and then fold it over with their legs to create a large parabolic reflector. Burrowing mole crickets build subterranean tunnels with a sound chamber close to the entrance that opens to the air in the shape of a trumpet. Their sound that passes through this structure is thus amplified considerably.
Male grasshoppers also stridulate, but instead rub the rough area on the inside of their hind leg over the forewing. Bladder grasshoppers have gone a step further. The male has an enormously enlarged balloon-like abdomen filled with air which serves as a resonance box to amplify his sound, much like a drum.
However, the champions of noise amongst the Kruger’s insects must be the cicada. Also called the Christmas beetle (although it actually isn’t a beetle, but a bug), the cicada is responsible for the loud and piercing strident shrill that is the ubiquitous sound of summer in the bushveld. Cicadas go about it differently, though; they have a membrane located on either side of their abdomen called a tymbal connected to an internal muscle that, when contracted, pulls on the tymbal making it ‘click’ in the same manner the metal lid of a tin will click when pressed. The tymbal then bounces back to its original position under the force of its own elasticity ready to be ‘clicked’ again by tugging the muscle. Amazingly, this repetition causes the tymbals to oscillate with their own momentum and need only be pulled every now and then to maintain the movement.
There is so much more to rhapsodise on the ways that insects make their sounds so I may return to the subject in a future blog. Next month I’ll head back to the mammal class and expound on a horse that lives in a river. Watch this space…
