Ripper
The Ripper (Minoris omniphagea) is a type of Tyranid organism which, is essentially a small, worm-like body with a mouth with rows of sharp teeth, and not much else.
Rippers are generally deployed in the later stages of a planetary invasion, when the Tyranids' hold over a planet is ensured and "tyranoforming" starts in full force. They will eat the planet's native life-forms (and Tyranid casualties, if need be) and once they have gorged themselves they crawl back to the digestion pools and jump in; there, they and the biomass they have consumed get digested, broken down, and later slurped up the capillary towers to the hive-ships above. Note that Rippers lack digestive systems themselves; the swarm is nothing if not efficient, and Rippers are only equipped with the minimal organs required to retrieve biomass and bring it to be processed. Consequently, they rely solely on the hive mind to do all the thinking for them and are highly unlikely to survive being seperated from the Hive Mind (e.g. through the death of a synapse creature). However, there have been recorded cases of Rippers reacting to the withdrawal of the Hive Mind by burrowing underground, where they use the biomass they've accumulated to metamorphose into just about any other kind Tyranid bioform, from a Hormagaunt to a Hive Tyrant.
In a nutshell, this means that the Tyranids regularly eat their own babies. So grimdark.
While small, Rippers can still be a formidable threat to most infantry in large numbers, where they'll drag down and chew the unlucky sods to death. Hell, there's even an instance where even tanks were overwhelmed by massive waves of Rippers that were spawned by the Parasite of Mortrex. The fact that they don't balk at the sight of tanks rolling over them make them an excellent tarpit, provided a synapse creature is nearby keeping them alive or someone else isn't bringing out templates or instant-death weapons, in which case, they'll be nothing more than a minor annoyance.