Hydralisks and roaches have a special melee animation when the enemy is within melee-range. I thought this was just for show, and I've actually heard speculation that it's a bug, but Oak recently mentioned that this attack shares some of the other benefits of melee-attacks (namely, not being affected by the guardian's shield).
However, I wouldn't think their attack would be affected by melee-upgrade, since there is no listing for it on the unit-card. So which is it? (references please, I'm not looking for speculation)