Cicadas have prominent eyes set wide apart, short antennae, and membranous front wings. They have an exceptionally loud song, produced not by stridulation, but by vibrating drumlike tymbals rapidly. The earliest known fossil Cicadomorpha appeared in the Upper Permian period; extant species occur all around the world in temperate to tropical climates. They typically live in trees, feeding on watery sap from xylem tissue and laying their eggs in a slit in the bark. Most cicadas are cryptic, singing at night to avoid predators. The periodic cicadas spend most of their lives as underground nymphs, emerging only after 13 or 17 years, which may reduce losses by starving their predators and eventually emerging in huge numbers that overwhelm and satiate any remaining predators. The annual cicadas are species that emerge every year. Though these cicada have life cycles that can vary from one to nine or more years as underground larvae, their emergence above ground as adults is not synchronized so some appear every year.