Let's see:
Warriors: melee fighters with tunnel capability
Blister Bugs, Blaster Bugs, Firefries: shooty types with minor variations in range and power, but slightly more fragile than Warriors and can't tunnel
Hopper Bugs: flying melee troops, *very* fast
Tanker Bugs: big, slow, super-tough melee monsters the size of a tank, armed with very powerful acid spray and able to tunnel
Plasma Bugs: really big, really slow walking plasma cannons, probably most useful for air defense
Brain Bugs: expensive but give extra actions to other Bugs each turn, as well as having scary psychic powers, fairly tough to kill but big penalties if they do drop
I think that's everything so far. I can see how to adapt most of this stuff to Defiance easily enough, but I'm a little reluctant to just skip the tunnel rules altogether, since it's (for me, at least) a big chunk of the Bug's flavor. Might be able to simulate a lot of it with the Covert Ops rules, though...have to reread that section first.
OTOH, I might just say the heck with it and just do the Bugs as a random gribbly army, which would let me just play them as a Matrix/Organic swarm force, as well as letting me add my own weird scratchbuilds. Have to do something with all those spare legs, I'll have 98 of them when I finish this latest batch...
Rich