So I had an Idea. Technically a bad Idea but that's just a factor of when I had an Idea.
[The Design Process]
To the Zerg, challenges were to be overcome. A novel stimulus allowed for new growth in the solving and bred out weakness in the Swarm. The current circumstances were less than ideal but not truly novel. Nonetheless, the Cerebrate pondered.
The problem: Terrans and their aircraft were present.
The complication: zerglings could not fight aircraft.
The solution: grow strains that could.
A rapid review of the two templates at hand was accomplished, leaving nothing unnoticed.
The problem: hydralisks required vespene in order to develop their thorax spine launchers.
The complication: vespene was complicated to produce and lesser in quality at this time, limiting its availability.
The solution: reduce reliance on vespene.
An obvious solution. Difficult, as the vespene catalysis was required to produce projectiles at a useful pace for the primary air defense of the Swarm. Reducing that capability would fatally compromise the strain's purpose.
The solution: make a new strain that was not meant to engage in direct combat.
Terrans were deeply flawed beings. Their soldiers would flee from personal danger and could only rarely agree on a course of action. While this would sometimes be a net positive on their combat performance it opened up many avenues of attack. If attacked where they thought they were safe the surprise often was their doom. Here, bereft of resources and with an unknown number of enemies, the Swarm needed to fight as well as it could.
Simply transferring hydralisk weapons onto a zergling was ineffective. The muscular foot of the hydralisk gave the launchers a range of motion that the low slung, quadrapedal zergling simply could not match. Worse, the larger torso was painfully mismatched on the smaller frame.
Cutting the number of spines ready at one time made it more manageable, taking the place of the cutting arms of the base strain. However, the ready ammunition was then pitiably small. A concern, even for a skirmish focused strain such as this.
A thought beckoned. Why use the hydralisk launcher at all? Redesigning the cutting arms to instead throw longer, thinner spines with interior channels for venom would neatly sidestep the need for advanced resources in production. Potential upgrades could add that back in in the form of self guided projectiles... But such a weapon would be impotent against all but the lowest flying aircraft. And here and now that capacity was needed.
Setting aside the cost effective throwing arms, the hydralisk launchers were once again considered. Range of motion would be gained by having a single propulsive channel on a pair of modified arms with peristaltic tubes supplying them with new ammunition from a dorsal growth sack. This would limit the number of projectiles that could be thrown at once, compared to larger organs placed directly onto the back of the zergling and mostly reliant on body positioning for aim.
A handful of exploratory spawns began harvesting combat data on the prospective strains. The arm mounted launchers were found to be superior as the scale of the conflict grew as they could fire while running with sufficient accuracy at large angles off bearing and could keep doing so for longer than their opposites before running through their limited stock of projectiles.
Interest grew, and more of the Cerebrate's attention was drawn to the project.
The new strain was meant to be a harassing agent and a desperate, early defense against airborne threats by a new hive cluster. Carapace could then be shucked away in exchange for longer limbs, a wiry build, and in general an ironic reversal of the Zerg changes to the original genetics. Adaptations for survival in the heat of a melee and charging a firing line were removed for the speed needed to dictate engagements against ground targets and respond appropriately to air attack.
A desultory designation of "spineling" was applied to the new strain, as pride began to well within the Cerebrate. Streamlining came next. A very slow gestation was began and the psychic power of the Cerebrate delicately threaded through the embryo to monitor how it grew in order to see what needed to be changed. Light carapace grew too early, stifling development of the body. Bones hardened too late, the spine pouch needed refinement...
Alone in a barren land the Cerebrate hummed to itself, content for the moment in the pure joy of creation.