Vermin can be any insects, arachnids, other arthropods, worms, and similar invertebrates, including giant versions of normal creatures. All vermin have darkvision and are immune to all mind-affecting effects. Most types of vermin are mindless.
Abilities (all already described in the MM) to make them more realistic and challenging:
Tremorsense. Many insects react to movement and vibration as much or more than visual cues. Giving a giant insect on the ground the ability to sense movement around it, even by invisible creatures, would certainly be reasonable (the ankheg, a big buglike thing, already has it, for example). Flying giant insects should probably have blindsight to reflect that they can feel movements in the air around them.
No Discernable Anatomy. This basically means no crits or sneak attacks, like the ability possessed by oozes and undead. You can cut the head off of a cockroach and it will die, but not from the loss of its head -- it will starve to death because it has no mouth. Other insects can be chopped in half, or lose a whole segment, and continue living. While insects aren't like oozes in that they do have body parts, they aren't like mammals, either, with vulnerable vitals, the loss of which spells near-instant death. A way to reflect this in-between nature could be in the form of a critical/sneak attack avoidance check. Give the giant insect a 50 percent chance to ignore the additional damage from a critical or sneak attack because while it was a strike to a vital area, they can still function normally without it. (A ranger's favored enemy bonuses should not be affected by this.)
These abilities, used together, would raise the CR of the giant insect by +1 (or one step for creatures with a CR less than 1, so CR 1/3 becomes CR 1/2, CR 1/2 becomes CR 1, and so on).