Heart of the Swarm- Evolution Guide: Explosive Results

Corrosive Acid: Your banelings deal double damage to the primary target of their explosive attack.

One of the issues sometimes faced with banelings is that if you’re attacking a cluster of marines with a tank in the middle, as an example, enough banelings to kill the marines by splash damage may not kill the tank, forcing you to invest in a fair number of extras.  This evolution aims to take care of that by adding focused damage on top of the usual splash.  This doesn’t work terribly well with splitters, whose spawn often strike the nearest thing that catches their eye (not necessarily what you wanted exploded most), but combines impressively with hunters, who will pounce and obliterate a central target, then spread out to take care of anything that didn’t just get splashed to death.

Rupture: Your banelings’ explosion radius is increased by 50%.

This, on the other hand, tackles one of the other issues with banelings, namely that a thick enough coating of inexpensive units will often keep them from damaging the thing you most want dead at all- again requiring extra banelings to obliterate it.  This goes beautifully with splitters, whose explosions will reach much farther and do far more overall damage, clearing the way for their spawn to rush into the gap and mutilate the units on the far side of the enemy regiment.  This is notably less valuable if you have hunters, particularly if you micromanage your banelings, as the hunters would have pounced the far side anyway if you’d just told them to and aren’t you thoughtless for not coming up with that on your own?  See if they ever buy donuts for you on a lark again.  Still loads of fun against masses of weaker enemy units, though.

Regenerative Acid: Your banelings’ explosions heal your units in addition to damaging enemies.

This is possibly the most amusing baneling evolution choice.  In masses of banelings, this will cause the death of one baneling to improve the chance of all its friends reaching what they’re supposed to blow up on.  In mixed melee forces, it will keep your other melee units up longer to an almost silly degree if you are, for instance, mixing banelings with ultralisks and performing a move-assault.  This effect is especially impressive with splitters, who will explode, heal their friends, split in two, continue the attack, and heal their friends twice more (possibly while assaulting the enemy).  Hunter banelings, on the other hand, tend to leap their targets from afar and will likely not catch allies in their explosions, so if you’re leaning in a hunter direction rather than a splitter direction, you can probably ignore this option entirely- it’s just not going to be as effective.