The Cicada's Song

FOCUS ON NATURE It's No Time for Insects When the Cicadas Sing

Cicadas are singing in the leafy cottonwood canopy above Cienega Creek. Already at 11 A.M. it's hot, miserably hot, and I'm plotting the next move of my camp stool into the deeper shade at creek side.

Overhead the cicadas buzz incessantly. It's mid-June; no rain has fallen in three months, and the monsoons, if they come this year, are weeks away. A hot, dry wind kicks up. The cicadas sing. I check my thermometer: 96° F. in the shade, heading past the century mark for the seventh day in a row. No birds call; nothing stirs in the ground litter. But hidden among graygreen leaves of cottonwoods high overhead, increasing many decibels in volume, it seems, with each degree of Fahrenheit, the singing of cicadas rises to a strident chorus.

The "singing" is actually a pulsing, high-pitched buzzing, more nearly drumming than singing, produced by a couple of platelike vibrating membranes in a cicada's thorax. And the two-and-a-half-inch-long singer I'm tuned into, a big guy as these insects go, is the dog day cicada. His scientific name is Tibicen canicularis, literally "tibia player of the dog days," so named, no doubt, when it waswidely accepted that cicadas produced sound by rubbing their forelegs together.

My use of the masculine gender here is no grammatical lapse: only male cicadas buzz, a fact that inspired Greek historian Xenophon to observe, "Happy are the cicadas, for they have voiceless wives." The male cicadas' singing signals their availability to females. Sometimes the males even fly around a bit, buzzing all the while, until they've attracted mates.

Meanwhile, female cicadas also may be airborne, homing in on calls of males. After mating, female cicadas insert rows of fertilized eggs just beneath the bark of slender twigs.

Mind you, all this is going on during fierce heat and drought, the "dog days," which arrive early in Arizona's deserts. Birds, rodents, lizards, and other insects retreat to shady copses or into underground burrows to escape the broiling heat. But cicadas, if anything, are spurred toward greater activity. How do they do it without overheating and dropping dead?

The answer to the cicada puzzler came to me by luck months later. I happened upon an article in Natural History by Eric C. Toolson that discusses research by himself and entomologists at Arizona State University on evaporative cooling in desert cicadas.

Toolson and the ASU researchers found that cicadas extract water from their blood and move it through a system of ducts out to their body surfaces where it evaporates and cools. Like humans, in other words, they sweat. In this way, some cicada species are able to reduce their body temperatures by as much as 10 to 15° F. below the surrounding air.

This sweating takes a lot of water, up to 35 percent of a cicada's body stores. But by using water sucked from xylem, the moisture-conducting tissues of vascular plants, cicadas replace lost fluids. It's an amazing adaptation to heat. Most insects cannot endure losses beyond 20 percent. Meanwhile, the cicada replaces water lost through sweating merely by sucking on moist twigs; in fact, it has water to spare. For a long time, cicada researchers have reported a fine spray beneath trees full of cicadas. Now they know that the mist is really surplus water excreted by cicadas.Okay, that answered my first question, but raised another: what possible evolutionary advantage is there for cicadas to mate and reproduce in searing heat, all the while singing their little thoraxes off? Oddly, the answer may be that a lot of creatures find these chunky insects quite tasty.

Early one cool summer morning, I watched a curve-billed thrasher repeatedly bash a cicada against a rock before gulping the morsel for breakfast. Ringtails and racoons eat them, so do snakes and lizards, and there's even one species of wasp that preys on them.Other animals have learned silence or stealth to avoid being found out and eaten by predators. But not the desert cicada. Instead it developed an ability to sweat, rare in insects. Thus, during periods of extreme heat when most would-be predators are holed up conserving their energy reserves and water stores, cicadas remain prodigiously active, singing, buzzing around, copulating, and laying eggs.When the eggs hatch, the nymphs drop to the ground and burrow among the roots of trees. In the Southwest, cicadas live as nymphs for up to three years, not the 13 to 17 years of some of the northern species, then emerge at night to climb onto the trunks of trees where the last metamorphosis takes place.Flexing its body, the nymph splits its hard shell and begins to crawl out. Then, very slowly, it begins to unfurl its wings, which lie like tightly folded buds atop its thorax. As the wings open, petal by diaphanous petal, gradually becoming rigid, the cicada abandons its old skin entirely. By dawn it's ready to fly higher into the trees and begin the cycle of singing and mating anew.