Chirping is only done by male crickets. Males chirp to scare away other males, to attract and impress females for mating, and to celebrate successfully mating with a female.
A chirp is not created with a cricket's mouth. The sound is created with the insect's wings using a process called stridulation. The bottom of each wing has a vein with comb-like serrations. A cricket will open his wings and then rub one wing along the serrations of the other wing. This rubbing creates the chirping and opening the wings helps project the sound.
Chirping & Temperature
The amount a cricket chirps depends upon different factors like species, age, and temperature. Temperature plays a role in the amount of chirps because crickets are cold-blooded and share the temperature of their environment. Crickets require energy to rub their wings and chirp. Energy is created by chemical reactions in the insects. Since chemical reactions occur more rapidly the warmer it gets, temperature controls the amount of energy available to chirp. The warmer it is the faster crickets can rub their wings and chirp. The colder it gets the slower they chirp.
Amos Dolbear was a physicist who discovered a correlation between a snowy tree cricket's chirps and the temperature. In 1897, he published an article with a formula to determine the temperature based upon the number of chirps a cricket makes in a set amount of time. This formula became know as Dolbear's Law.
The formula for the temperature in degrees Fahrenheit is:
Temperature °F = 50 + (chirps per minute - 40)/4
A simplified version only requires counting chirps for 15 seconds:
Temperature °F = 40 + chirps per 15 seconds
The formula for the temperature in degrees Celsius is:
Temperature °C = 10 + (chirps per minute - 40)/7
A simplified version only requires counting chirps for 8 seconds:
Temperature °C = 5 + chirps per 8 seconds
The simplified formulas may not be exact, but will be close to the actual temperature. Using the chirps from field crickets, house crickets, and other types of crickets may not be as accurate due to other factors like mating and age, but will give a close estimate of the actual temperature.