Patrick Harper's Maggots were first introduced to the Sharpe series in Sharpe's Eagle.
Harper administers maggot treatment.
They were used to clean Sharpe's thigh wound, which was given to him by a French Hussar (Sharpe's Eagle : Chapter 1) during a solo charge against Sharpe and Harper whilst they were out escorting Captain Michael Hogan, who was engaged in engineering and reconnaissance duties.
The maggots, which were usually kept in a tin, ate away a wound's dead tissue, allowing the remaining healthy flesh to close naturally, thus stopping the wound from going septic and killing the patient; "the blood poisoned, the flesh discolored and stinking, and the doctors helpless to do anything but let the man sweat and rot to his death in the charnal houses they called hospitals." (Sharpe's Eagle : Chapter 1)