||The alien probe which launched a devastating attack on earth was, initially at least, a total mystery. No trace of it's area of origin could be found, and no clue as to who had sent it.
Fortunately, the mystery was soon resolved at least in part. Information from the Suliban indicated that a species called the Xindi had sent the probe. The NX class Enterprise was sent to locate the Xindi in a region of space called "The Expanse", a dangerous area filled with anomalies.8
The Xindi proved to be not one but five separate intelligent species, all of which had evolved on a single planet.9 The five had evolved a complex relationship over time, with alliances forming and shifting constantly. Wars between two or more members were not unknown, and indeed one such war had resulted in the destruction of the Xindi homeworld - and the extinction of a sixth branch of the species, the Avians.10 The remaining five were :
Aquatics : The Aquatics were one of the few known sentient species which were more comfortable underwater than on land. They were somewhat mysterious, often holding back and letting the other species make decisions.9
Insectoids : Resembling a Humanoid version of an ant, the Insectoids were typically arrogant and aggressive. They were one of the groups which pushed strongly for the attack on Earth, and on the Enterprise during it's mission to make contact with them.9 Insectoids live for only about 12 years. They are genderless, and all Insectoids are capable of laying eggs. The eggs have an unusual defence mechanism - they can spray a substance on anybody who approaches which causes the person to become 'imprinted' on the eggs, developing a highly exaggerated sense of protectiveness towards them.11
Primates : The most closely Humanoid of the Xindi, the primates were also less overtly aggressive than some of the others but did tend to be more cunning and somewhat devious in nature. They were also one of the driving forces behind the attack on Humanity.9
Reptilians : Rivalling the Insectoids for aggressiveness, the Reptilians tended to be paranoid to the point of Xenophobic. It was the Reptilians and Insectoids who detonated the weapons which destroyed the Xindi homeworld at the end of a war which had lasted for a hundred years.10
Arboreals : The Arboreals were one of the more Humanoid of the Xindi. Rather hirsute in appearance, the Arboreals tended to be more peaceful than the Reptilians and Insectoids. Some of the Arboreals proved to be allies of the Human effort to end hostilities with the Xindi.10