Edit: Come to think of it (and this is completely off the cuff), perhaps these two different classes actually correspond to sixes? I just realized that a fertile Kwama queen technically consists of both a worker and a forager. If that makes any sense to begin with, I wouldn't know which one would be the male and which the female, though.
In real-life insects of this type (order hymanoptera), it is the females that carry the poison. The reason for this is that the poison production/delivery system is a modified ovapositor. Most insects use this for depositing eggs wherever they deposit their eggs. Wasps continue to do it that way, depositing both eggs and a toxin into soft-bodied prey, which is then deposited in a safe place, generally a nest of some sort. The larva hatches, lives off the insect until it pupates right there, emerging in advlt form. The males never eat insects again after this point; they live primarily off nectar after that. Actually, that's a common theme in the insect world: it is only the female that needs protein in its advlt stage, for egg production.
However, for communal hymanopterans, like ants and bees, the ovapositor has evolved beyond this use. Only the queen actually lays eggs among ants and bees, while all the rest use it for combat purposes only. This, by the way, is why all the workers of these species are females: males simply do not have the hardware. Interestingly enough, this device evolved out of existence entirely among certain species. Single node ants have no stinger!
As for kwama, scribs must be female, foragers male. Scribs are the only single that carry a toxin (paralysis), which must come from their modified ovapositor. Males, on the other hand, have more conventional weapons, as well as an aggressiveness that serves the forager's predatory purpose. Warriors are a compound creature, combining the toxin producing ability of the female with the aggression of the male. Queens are also a compound creature, though in this, the male is subordinate to the female, acting purely for fertilization purposes. It is unknown whether a queen keeps a single mate for her entire lifetime, or if males periodically die and are replaced.
Of course, there is another possible theory, in which the toxic attack form is best explained by a real-world anology to order hymanoptera, but is rather somehow related to Muatra.