FT1 is a buffer amplifier, TR1 and TR2 are a differential amplifier. as a positive (note: this circuit only detects positive electrostatic charges) voltage is applied to the gate of FT1, FT1's source goes more positive, making TR1 conduct. the gain of TR1 is effectively doubled by TR2 acting in an opposite manner from TR1. increased conduction in TR1 lights the LED.
R1 is a very high value, because static voltages are in the multi-thousand volt range, and only 0.7V is needed to turn on TR1, so R1 becomes part of a voltage divider with either FT1's gate, or R8, reducing the voltage, and limiting the current across FT1's g-s junction. this circuit is very similar to what is called an "electrometer".