According to the Miner's Manual, the current leading theory regarding Bulk Detonators is that they represent a later, rarely-reached stage in the life cycle of the Glyphid Exploder, Displodia communalis. An anatomical examination of both creatures and the application of logical thought, however, suggests that this is not truly the case.

Anatomical Differences

To begin with, D. communalis shows notable anatomical differences from the Bulk Detonator; D. communalis possesses 6 superior teeth and 8 inferior teeth - four on each mandible. The Detonator, however, possesses 5 superior and 14 inferior teeth - 9 on the right mandible and 5 on the left. The presence of combustive bladders on the six dorsal limbs is also different - the Exploder has bladders on all but the medial dorsal limb on the right side, while the Detonator only lacks bladders on the same limb of the left side. It is possible these differences are simply a part of D. communalis' natural development but, given the nature of Detonators, it is hard to believe that it's growth would result in the loss of any characteristics.

Exploder Reproduction

Given the extremely short lifespan of Displodia communalis, it is highly likely that the creatures reach reproductive maturity at an early age; there is no way the species could be so prolific otherwise. Considering this, it is unlikely that there is any real selection pressure on members of this species to survive a long time - we have seen nothing to suggest that Glyphids spend much time at all caring for their young.

There is no reason that D. communalis would continue to develop past this point - it would merely be a waste of energy - and they certainly would not develop new the dramatic new abilities we see in Detonators. The fact that we have never seen any specimens in the intermediate stages of development - larger than the mature exploders we regularly encounter but smaller and less developed than any Detonator - lends strength to this argument. If exploders continued to develop after reaching sexual maturity they would not all be the same size.

Radioactive Exploders

The final nail in the coffin of the "grown exploder" theory is the existence of Displodia iredescentia, the Radioactive Exploder. D. iredescentia is a mutation of D. communalis - the nature of their explosions and relative resistances and weaknesses to suggest that they are a different but closely related species. They share the same explosive fluids which, according to the Miner's Manual, "are highly reactive to gamma radiation," giving D. iredescentia its distinctive radioactive glow.

Notably missing, however, are any radioactive Bulk Detonators. If Detonators were truly just more-developed Exploders, then all of those encountered in the Radioactive Exclusion Zone would, in fact, be radioactive. No Detonator thus encountered has emitted radiation nor a green glow; as a result, they must produce a different set of chemicals to use in their explosive reactions.

Conclusion

With all of this in mind, I posit that Bulk Detonators cannot be simply over-grown Exploders, but are rather a different species of the same genus: Displodia giganticus (and the much rarer sub species: Displodia giganticus crassus). I believe that their rarity comes not from an inability to survive to maturity, but rather the slow crawl of natural extinction. We know from the fossils encountered in the Fungus Bogs and the Dense Biozone that over the past eons Hoxxes IV has selected for smaller and smaller creatures - that D. giganticus is the largest and rarest Glyphid encountered suggests they are merely the last vestiges of a dying species from times long past.