In the Twin Cities area the first annual cicada was heard calling on July 6 this year; last year it was on June 15. Annual cicadas are common insects, more often heard than seen. The males sing loudly from up in the trees, with a sharp high-pitched sound. Their pulsating buzzing intensifies with warmth. Some uninformed people attribute this persistent sensation to utility wires vibrating in the heat.

Annual cicadas are about 1.5 inches long, black in color with green markings and clear wings. They possess a pair of sound chambers, one on each side of the abdomen. The familiar sound is produced when the insect forces air over the membranes stretched over these sound chambers. Whatever the mechanism, many of us enjoy hearing the buzzing sounds on warm summer days. There are more than 75 species of cicadas in North America, each with its own special song.

Most cicadas have a life cycle of two to five years, with a fresh generation of adults appearing each summer. The female cicadas cut slits in young tree and shrub twigs, and deposit their eggs there. As the wingless young hatch, they drop to the ground, burrow in and stay for a number of years as nymphs, living on the juices sucked from roots. When fully grown, the nymph finally emerges from the soil and climbs a tree trunk. As he or she ascends, the skin splits down the back and the adult comes forth.

Jim Gilbert's Nature Notes are heard on WCCO Radio Sundays at 7:15 a.m. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.