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More recent fluff is those creatures can eat and digest to feed themselves, if we are to extrapolate on the snippet where a Termagamt is eating a dude. Hormagaunts are also noted as being able to asexually reproduce and grow to maturity on their own, which means nom noms. Overall it makes more sense logistically; organisms would ptherwise require separate organisms to feed them, or need to carry all the calories they'd potentially need from the onset.

Perhaps more importantly there is biting, which by numerous account Tyranids do quite often. The mouth is very much another weapon in itself. Sensory apparatus, on the other hand, are relevant because feeding individual organisms all the sensory input they need means they are even more effectively neutered by being severed from synapse organisms, already a potential weak point the Hive Mind would no doubt be aware of. The central nervous system being in the head of organisms IRL is so it can be next to all those senses and the large number of neurons connected to them. This is both more biologically efficient, relevant IRL, and in 40k those fractions of a second in reflex speed can very much matter.

Larger organisms having bigger skulls also makes sense, as their robust physiology both supports and necessitates thicker bone to protect its brain. The skull also needs to have a good degree of strength as a mounting for the jaw muscles, which we know are quite powerful. It is even theorized that during human evolution a mutation which weakened our jaw muscles allowed the skull to become thinner and prove room for larger brains--big nids don't need the larger brains but dem jaw muscles gotta attach somewhere!