This project demonstrates an acoustic feedback oscillator which has the ability to mimic closely the noise of the night insects. The sounds are controlled by adjusting the proximity of the earphone and the speaker.
The amplifier used for this project is an audio amplifier with two floors, with direct coupling. The floor of Q1 is biased by the "universal bias circuit", you notice the bias-based resistance that is between the C-E of the floor of Q2.
This arrangement does not give as many circuit stability that when a fixed resistor is used in place of Q2, but it shows another method of connecting a DC amplifier.
The signal characteristics are controlled mainly by the 3.3 uF capacitor, then you'll want to experiment with different values for it. The operation is controlled with the earphone to different positions near the speaker.