During this time, a cacophony of singing cicadas reverberates through the forest. There are a number of different species out there, and you can actually tell them apart by their sound alone (which is good, because how many have you ever seen?) Some species start singing in May, while others are still crooning well into the fall, but since the majority of Tibicen species are heard from mid-July to late-August, they are all called “dog day” cicadas.
Each adult dog day cicada lives only a few weeks, and spends its time sucking juices from tender twigs, mating, and producing hundreds of eggs. Those eggs take a month or so to hatch. The hatchlings, called nymphs, then live underground for up to a decade. These cicadas are not "annual" at all, but since every year is somebody's tenth birthday (or hatchday), we can hear dog day cicadas buzzing in the treetops every July and August.
If you are interested in reading more about the astronomical dog days, check out Wikipedia, Cornell University’s Astronomy Department, and this guy’s website.
To find out which cicadas are singing when – and to hear their songs – check out the University of Florida’s Institute of Food and Agricultural Service, and the University of Connecticut’s Ecology and Evolutionary Biology Department. Other good cicada sources include Wikipedia, Cicadamania and this guy's website.