Arachnid Aliens are Facehuggers that never latched onto a host.The Xenomorph embryo inside of them starts to break down and the embryo are fused together making a Xenomorph head. The Arachnid species is also known for having two front claw-like legs and four "facehugger" legs. Although not necessarily derived from a spider, they look like the fronts of spiders. The tail of the facehugger eventually grows into a tail of that of a Xenomorph which is used to impale their victims. It is not known how a genetically grown Arachnid Alien would look like, but there are many speculations. This Arachnid Alien would have a large abdomen that can spin "black secretion-like webs", like a spider would. The webs create a unique type of hive, which usually covers long dark hallways or cover large rooms.
Unlike other Xenomorph breeds, Arachnid Xenomorphs can't swim. This is possibly because their bodies are not designed to do so, and-or that the Facehugger's 'breathing sacs' had mutated into lungs, keeping it alive instead of its non-existent host.