Crickets, family Gryllidae (also known as "true crickets"), are insects somewhat related to grasshoppers, and more closely related to katydids or bush crickets (family Tettigoniidae). They have somewhat flattened bodies and long antennae. There are about 900 species of crickets. They tend to be nocturnal and are often confused with grasshoppers because they have a similar body structure including jumping hind legs. Crickets are harmless to humans.
Crickets chirp at different rates depending on their species and the temperature of their environment. Most species chirp at higher rates the higher the temperature is (approximately 62 chirps a minute at 13°C in one common species; each species has its own rate). The relationship between temperature and the rate of chirping is known as Dolbear's Law. According to this law, counting the number of chirps produced in 14 seconds by the snowy tree cricket common in the United States and adding 40 will approximately equal the temperature in degrees Fahrenheit.
The sound emitted by crickets is commonly referred to as chirping; the scientific name is stridulation. Only the male crickets chirp. The sound is emitted by the stridulatory organ, a large vein running along the bottom of each wing, covered with "teeth" (serration) much like a comb. The chirping sound is created by running the top of one wing along the teeth at the bottom of the other wing. As he does this, the cricket also holds the wings up and open, so that the wing membranes can act as acoustical sails. It is a popular myth that the cricket chirps by rubbing its legs together.