Which reaper ground forces derive from which races?
This is all explained in the in-game codex. Going from memory:
- Husks: Human
- Cannibals: Batarian (and human, see FAE's comment)
- Brute: mix of Turian and Krogan
- Banshee: Asari
- Ravager: Rachni
- Marauder: Turian
- Harvester: Harvester :p