Have you ever noticed how easy it is to shut out some sounds - the air conditioner, the fridge, the traffic noise? Sometimes, however, sounds can be so intrusive that they cannot be ignored.

Such is the sound of a cicada that is calling, especially if it is nearby. The loudest of all the insects, the cicada can whine at 4000-7000 cycles/second at ear-splitting intensity. This modulated train of pulses is thought to be species specific. Male cicadas use two thin membranes, called tymbals, located on each side of the first segment of the abdomen to produce their sound. When the tymbal muscle, attached to the inside of the membrane, contracts, the membrane buckles producing a click. Each membrane is strengthened by a series of ribs the presence of which causes a full contraction to produce a series of clicks. Other muscles create additional deformations of the membranes thereby modifying the sound. Cavities behind the tymbals resonate at frequencies which are characteristic for the species. Other body structures as well as the calling posture also affect the sound. When singing the male cicada protects its ``ears'', the tympanic membranes, by contracting them.

Cicadas seem to prefer dryer habitats such as deserts, vegetated hillsides and forest clearings. Females lay their eggs in slits cut in the vegetation with their sharp ovipositors. Normally damage to vegetation is not serious but cicadas have from time to time reached pest status on corn, date palms, cotton, asparagus and other commercial crops. After the eggs hatch the nymphs fall to the ground. Most of their lives are spent as larvae buried underground sucking on the juices in plant roots. When the nymph is mature it tunnels to the surface, finds some nearly vertical structure, climbs up and sheds its skin, emerging as the insect we are more familiar with - one that makes lots of noise. The full life cycle probably averages 2-5 years.

Most cicadas are not periodical. The massive cyclic emergences that people from the east and midwest talk about consist of individuals of 3 species of cicada all of which have life cycles which last 13 and 17 years. In a given area all the species have the same development time and emerge during the same short period. Vast numbers, especially of the periodical cicadas (the others are more wary), are consumed by birds attracted by the noise. The calls also attract parasitic flies. Others are caught by the Cicada Killer Wasp which leaves the paralyzed victim as food for it larva.

About 1600 species of cicada exist world wide. While some 65 species have been recorded in California and at least 15 in the San Diego area, I have never heard a cicada at NIC (any hearings would be appreciated although I don't expect them here). If you happen to be in an area where the cicadas are singing, try to locate one. The hunt will test your patience and powers of observation. The one I watched singing was a real treat. In the meantime, around NIC, listen for the moderately high pitched trilling of the tree cricket, another insect singer, whose song, unfortunately, often goes unnoticed because of the traffic noise.

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Ron Lyons (volunteer 1990-1999)
Chula Vista Nature Center, 1000 Gunpowder Point Drive, Chula Vista, CA 91910-1201