What are some of the devices used in fugue to vary the main melody?

In a fugue, several devices are employed to vary the main melody, including inversion (reversing the direction of the melody), retrograde (playing the melody backward), and augmentation (lengthening the note values). Additionally, diminutions can be used to embellish the melody with shorter, faster notes. Stretto, where voices enter in close succession, and modulation, changing the key, also serve to create variation and complexity in the fugue.