Arachni is yet another MOC that is nothing more but a spontanic design. Based on leftover from badly-done Defiler, from which I liked only the legs. All the rest was just experimenting - maybe use insectoids' eye as a cockpit canopy? Or take Dreadnought's machine gun... why not? :D
The Arachni walker looks like it was heavily armoured, and indeed it is so. With thick armor, speed, agility, and ability to get where common tracked or wheeled vehicles can't, Arachni is valuable unit.