User blog comment:H-Man Havoc/New Squadmate Species for ME3?/@comment-1326762-20101231064238/@comment-743796-20101231163034

the problem is that every squadmate in the ME universe (so far) has met 2 basic, but major requirements: that they're sentient, and that they're a human/humanoid species. Rachni meets the first condition but not the second. The only way the Rachni would meet the second condition is by taking over a humanoid body, much like both Asari specimens from ME1 and 2 respectively. With that in mind, there are a handful species who could transition to a squadmate: Volus and Batarian.