The impressive radiation of woodlouse-grasses across Apterra's varied biomes has, as the name might suggest, brought along for the ride an assortment of interesting isopod companions. Many of these are quite unspecialized in their habits, feeding opportunistically on the secretions of their Bouteloua symbiotes but just as happy to seek additional means elsewhere. The traitors persist as well; though most of the herbivorous pill bugs have moved on to other, non-toxic varieties of Apterran grass, a few tough and mildly poison-resistant forms still bother the woodlouse-grasses and their mutualists from time to time. One pioneering branch of woodlice descended from the original cooperative forms has even ventured into the venerable profession of weed control, consuming any plants in its territory except its preferred host species. At the tops of the tallest palm-grasses, though, lives a species unlike any other so far.
A. arborealis is the only species of Apterran pill bug to fully cut ties with the earth. It dwells in the canopies of growing palm-grass "trees" and other giant woodlouse-grasses, finding its sustenance as its ancestors did, by drinking the sweet nectary sap produced by the very plant it calls home. In exchange, it scours the entire surface of the grass, peeking into every nook and cranny of its stems and leaves. Its diet is now so specialized that it rarely even eats the molds and fungi it finds; it simply tosses them to the ground. In many instances, another species of symbiotic isopod cleans the grass's roots, and these generalists are more than happy to consume such scraps. The arboreal forms, though, only eat one food source besides their daily intake of sap. This, in fact, is their true purpose to the plant, for it has now evolved its own antifungal compounds to keep most aboveground infections at bay.
All day long, after they've eaten their fill of sap, these pill bugs sit at the tips of each leaf, waving their antennae rhythmically. They're waiting to catch the scent of an invading phytophagous woodlouse, whose approach is always accompanied by the odor of freshly cut grass. Of course, this is a false alarm in most instances, for most of the grazing isopod forms steer clear of the toxic palm-grasses. Once in a blue moon, however, this scent heralds the arrival of an Armadillidium that isn't so easily deterred.
The arboreal isopods' enemies are not the naïve nectar-slurping moochers of millennia past. The descendants of these early parasites now travel in swarms, crossing swathes of pseudoforest floor in search of their next meal. Today, a group of several dozen is pooling around the base of the palm-grass, waiting for the rest of its numbers to arrive. When they reach a critical mass of a hundred or so, the parasites rush up the smooth stalk of the plant, only to be met with a wall of spines and powerful mandibles. The defenders' bodies are perfectly designed to repel this army; their heads are flat and wedge-like, capable of sliding beneath their opponents and flinging them back to ground level. Just behind this tossing apparatus is a pair of curved spines originating from the first body segment, used for prodding at the eyes of other isopods. Their last three pairs of legs have evolved to become bulkier, capable of holding fast to their home and holding the line even in the face of the parasites' superior numbers.
Together, the A. arborealis make gradual progress, pushing the invading force groundward in pulsating advances of a few millimeters at a time. When they reach the soil, they make sure to chase their cousins some distance away, only satisfied once they see the army marching off in a different direction. Despite their seemingly flawless victory, they came closer to failure than one might realize from watching the fight. If the parasites had numbered just a dozen or so more, the outcome could have easily swung the other way. Both the mutualists and their adversaries rely on their numbers for survival. The parasites operate as a relatively-disorganized swarm, attempting to overwhelm their foes. If one is thrown to the ground, it can climb up again to rejoin its conspecifics. Only if all are defeated can the whole be chased away. The arboreal isopods, on the other hand, depend on a more close-knit cooperative effort. If even a single invader breaks through their lines, their defense effort is doomed, for they are incredibly vulnerable from behind. When their strategy succeeds, though, the rewards are great. The ten or so grazers that were outright killed in the battle will satisfy their growing hunger for protein, providing the nutrition they need to birth another generation of their species.