The hive insectoids in tremulous is just a projectile with a bbox size of 0,0,0 that changes direction every 0.5 seconds, and has a particle system emiting from it that takes its inertia similar to the blaster particle system, and the particles are just two .tga files of the insects animating. The visuals do not do anything and the missile sacrifices itself upon contact with a buildable, player or <world> (the game map). It's a very simply version of a homing missile basically.
TL;DR: The hive insectoid is just an invisable single projectile that chases humans and emits visuals of insectoids.
For the spider nests, well i don't see why can't we just have a wallwalking-missile that chases its opponent. Although that would be silly to look at around corners though.
Then again, i don't know the capabilities of the UE...