The Skrit Na are a race of aliens from the Animorphs book series by K.A. Applegate. They are similar to the Jawas in Star Wars in that they scavenge for profit. They also capture specimens from the many planets they visit and either perform strange medical procedures on them or put them in a zoo on their homeworld. The Time Matrix was first discovered on a Skrit Na ship.
They have a life cycle similar to caterpillars and butterflies: they start as Skrit, which are similar to giant insects. After some time they weave a cocoon and change into a Na, which look like the famous "Grey" aliens reported by alien abductees. In fact, it is suggested that the Skrit Na are indeed the Greys, given that their appearance, activities and even their ship design is the same.
The Skrits seem to be relativately unintelligent,and are used to perform simple tasks on the ships.After some time they weave a cocoon and change into a Na, which look like the famous "Grey" aliens reported by alien abductees.
Due to their strange life cycle, SkritNa are considered poor hosts for a Yeerk, so the race was never conquered; rather, the Yeerks seem to have developed a kind of trading partnership with the SkritNa, not caring much that the SkritNa usually steal or abduct their loot.
The SkritNa are mentioned briefly by the character Father in The Ellimist Chronicles, where Father claims to have assimilated some into his being, meaning they were already space-faring by this point in the Animorphs timeline.