The circuit behaves like a true mosquito: once the light dims, the mosquito starts an annoying buzz. When its light again it becomes mute, so that, as in the case of genuine mosquito is very difficult to spot.
Voltage from pin 2’s IC1a determines if mosquito is active or not. The level of this voltage depends on the foto-variable-resistance R1 and adjustment of P1. It takes about 90 seconds until C3 will completely load and in this interval pin 6 of IC1b will be in the state H. Once the pin voltage falls below the lower threshold of IC1b, the exit gate (pine 4) passes in the state H. As a result, Ic1c start oscillates.
At the oscillator exit will be produced a rectangular signal that will be passed through IC1d buffer and then applied to buzzer. The buzzer will work as long as T2 is open and the buzz tone will not be continuously and completely random.
Once it is light again, the potential of pine 2 head of IC1a falls, which cause rapid discharge of C2 by Di and R4, so the oscillator locks immediately.The current that is determined through the circuit is only 2-5 mA, so if you use a 9V battery, it will have a duration of the operation long enough.