Yeah, they are. those worms i think probably extend through the limbs of the scarab.  This is not the first time the "lekgolo" have been used like this.  The lekgolo have had other functions then as hunters from before.  they are used in excavation and recovery because they eat certain metals (I can't find my Contact Harvest book, but i'm pretty sure they were eating away metal debris, and even some forerunner stuff, which lead to their "taming" by the covenant, so that they would not eat artifacts! they were then used to clear out junk from the forerunner dreadnaught, which is definitely mentioned in Contact Harvest). 

 I do think Halo 2's scarab however is mostly mechancical.  I'm not a big fan of the theory that they are mining vehicles, but with Halo 3 i suppose this is more likely.  anyway, the Halo 2 scarab clearly has a cockpit area, while the Halo 3 scarab does nto seems to have any cockpit or panels into the vehicles, unless of course it is just not initially obvious.