[1] Its size is an example of island gigantism,[2] it being the largest known pill-millipede in the world, with some individuals reaching a maximum length of 90 mm (3.5 inches) long.

The anal shield in males is strongly bell-shaped and covered laterally with numerous small round pits, some of which support a short hair.

The anal shield in females is weakly bell-shaped and covered laterally with numerous small round pits.

The stigma-carrying plates in females consist of a short lobe covered with long hairs and curved towards the coxa.

[3] Z. neptunus can be found living among leaf litter in moist lowland rainforest habitats up to elevations of 850 metres above sea level.

[5] Captive populations tend to have a scarcity of males, possibly because their smaller size renders them less desirable to collectors compared to the larger females.

[3] The species is known to have a short lifespan in captivity, often starving to death due to conditions incompatible with its regionally-specific metabolic adaptations.