Читать книгу Extreme Insects - Richard Jones - Страница 29

NAME	dog day cicada Tibicen pronotalis
LOCATION	North America
ABILITY	makes the loudest noise of any insect

Insects are generally small, secretive and quiet. Most are reluctant to draw attention to themselves, but the cicadas are an exception. Along with crickets, katydids and grasshoppers, the cicadas use sound to communicate with each other, and they do so in the loudest manner possible.

On each side of the first abdominal segment is a large round organ called the tymbal. The tymbal, just like a drum, has a stiff elastic membrane held taught by a rigid circular frame. Inside the insect’s abdomen, a large muscle is attached by a narrow thread to the centre of the membrane. When the muscle contracts it distorts the tymbal membrane, causing it to buckle suddenly, creating an audible snap. When it relaxes, the membrane clicks back to its resting position. By vibrating the membrane at 4,000 to 7,000 times a second, the clicks become merged into a continuous whining buzz. Inside the abdomen, two air sacs (modified breathing tubes) are tuned to the natural frequency of the tymbals and act as amplifiers. The noise made is astounding, easily competing with loud power tools, lawn mowers or motorcycles. Cicadas on the motorway verge can often be heard from inside fast-moving cars, or through dense forest from over 1 km away.

The volume of a noise is measured using sound pressure level meters. The loudest sustained volume recorded for an insect is for an African cicada, Brevisana brevis, which clocked up 106.7 decibels. Human hearing is damaged by prolonged exposure to this volume and the recommended limit is less than two hours per day. Brevisana keeps it up all day long. The loudest peak cicada call ever recorded was for one of the North American dog day cicadas, Tibicen pronotalis, which reached 108.9 decibels during an alarm call. The normal purpose of cicada ‘songs’ is for males to call to females and announce territoriality to each other. On the whole the largest cicadas make the most noise, so everyone knows who is the biggest. Alarm calls are made as a defence against birds, and at these volumes the sound is truly repellent.