The Plague is a very old family of woodlice, first emerging as grazing forms that terrorized the tall tropical grasses that would later give rise to palm-grasses and skystalks. Along with threats from fungal decomposers, it was these ancestral isopods that prompted the woodlouse-grasses to evolve their mutualistic relationship with the Plague's arboreal cousins, changing the course of Apterran history. For a time, the latter partnership had the advantage, relying on toxins and well-organized defense to deter the herbivorous crustaceans. As time went on, however, the aggressors became stronger, gaining a high degree of poison resistance and tough exoskeletons to win the battle. In the Middle Muricene, when we last observed the fight between the Plague and the mycad it targeted, the two forces were almost equally matched. Like the multi-species army that protected the plant, the Plague at the time traveled alongside companions such as the reaper, a species of harpoonjaw mosquito whose descendants have since switched to easier food sources or to less aggressive strategies of consuming woodlouse-grasses. For many millennia thereafter, the Plague moved alone, relying on nothing but their numbers, strength, and the element of surprise to conquer woodlouse-grasses in pseudoforests and woodlouse-grasslands alike. Today, the genus Pestilarthrus is the most dominant in this niche, ranging widely across all of Panapterra, sub-Abeli, the Northern Isles, and even into Choeropical pseudoforests far off Ailuropia's west coast.

In the last 100,000 years, the Plague once again has gained a comrade, this time in the form of Bridgeworms (Dodecipes scalarius), a species of dodecipede that has abandoned the safety of its ancestors' leaf-litter home for the more active protection granted by its new, more gregarious lifestyle. Larger than any other member of their genus but also slower (for there is no advantage in outrunning the slower Plague woodlice that they rely on), they weave gracefully between the smaller bugs. They exist in low numbers, generally no more than one for every two or three hundred Pestilarthrus, as their function within the group only requires a handful of bridgeworms to succeed. In fact, since the mostly-defenseless giants serve no purpose to the swarm when not actively attacking a plant, excess populations are sometimes culled, especially during lean times.

This is not to say the two species are adversaries; such extreme measures are purely practical on the part of the Plague, and when invading a well-defended palm-grass their cooperative abilities truly shine. In ages past, the greatest barrier to reaching the delicious foliage at the top of the woodlouse-grass was the dug-in ring of sheriff isopods that encircled the stem, blocking all who attempted to climb higher. With their blunt, wall-like heads and backward-pointing spines, sheriffs are near-impossible to move from their lines, and ancient Plague strategies relied on gradually chewing their way through the barricade, taking advantage of the fact that the defenders had no choice but to remain stationary. This brute-force method was often successful, but its time-consuming nature allowed other arboreal isopod species, such as flipbugs or basal staghorns, and even gouger harpoonjaws, to swoop in and pick off the attackers, weakening their offensive and sometimes even turning the tide against the Pestilarthrids. Today, no such drawn-out conflicts take place; battles are quick and decisive, and the winner is generally determined in a matter of minutes.

When the Plague approaches a tasty-looking palm-grass (or even a particularly large and well-protected skystalk), four or five bridgeworms will ascend ahead of the swarm. Slower than their cousins but still more nimble than the cumbersome sheriffs, they crawl straight over the blockade, arching outward to avoid touching the sheriffs' upward-projecting spines. A small group of Plague woodlice soon follows, flanking the bridgeworm and fighting off sheriffs in the immediate vicinity to ensure no harm comes to their larger compatriot. The majority of the herbivores, though, crawl single-file up the dodecipede's body, pouring into the upper parts of the woodlouse-grass at a rate of dozens per minute across each bridge deployed. As with earlier fights, even a small number of Plague making it past the defensive line can spell disaster, and the situation is no different today; once the crossing is established, the plant has little chance of survival, and the two-species alliance is sure to quickly secure a meal. 

As both species' diets are composed of over 90% plant matter, the hundreds of carcasses left behind when they move on will soon become food for scavengers like Dermestemimids and plaguelings, the latter of which give the horde a brief reprieve from their usual constant harassment to take advantage of this easier food source. Some grasses, like splitstalks, canopy-palms, and especially palm-brush, may eventually recover from complete defoliation by growing new stems, provided at least a few of their resident woodlice survived the onslaught. In some regions, the defenders do have one remaining trick up their sleeves that is occasionally enough to defeat even the mighty bridgeworm. Recently evolved from the flipbug is the Bladetrap Bug (Platyarthrus viridis), a large, flat, oval-shaped woodlouse whose normal day-to-day life involves ridding its host plant of parasites like leechbugs, but which has in recent years become a valuable weapon against dodecipede intruders. It has a dark green coloration to blend in with the stalks it rests upon. To human eyes, theirs would be quite an unconvincing disguise, but the bridgeworms, who still have poor eyesight after millions of years living on the dark pseudoforest floor, find them almost invisible. This allows them to violently fling their enemies several meters away, preventing the formation of sheriff-spanning bridges and giving their cousins ample time to drive off the Plague. This is bound to result in an arms race as the epochs-long war continues, with bridgeworms becoming sharper-sighted and bladetraps gaining ever-more-convincing camouflage.