Trichorhina tomentosa, or more commonly known as the "dwarf white isopod" is potentially the most popular and useful isopod, but not for reasons that most would assume. They're very small, most adults reaching a size of 4mm, and have no interesting patterns. They are rarely observed, spending most of their time burrowed and only surface for available food sources.
Measurement of adult Trichorhina tomentosa.
So why do people get them? The answer lies in their size: they are a perfect feeder for small animals, and docile enough they don't pose a threat to animals in bioactive inhabitants. Dwarf whites are especially popular in the frog hobby; they are used as feeders for adults and froglets. Amphibians are commonly kept in bioactive setups because of their ability to thrive in natural environments, and dwarf whites are perfect for this for their role as a feeder, their quick reproduction, thriving in very humid environments, inability to threaten animals.
So, what is a bioactive setup? This method of keeping animals in terrariums has been rising in popularity in the last few years. The goal of bioactive setups is to provide an environment that is both low maintenance and enriching to the animals. A proper bioactive setup contains a drainage layer, substrate, leaf litter, plants, isopods (sometimes of different species and sizes), springtails and the star large animal inhabitant. Some people are so passionate about bio and plants they don't have any reptiles in a setup at all! Dwarf whites are the part of the terrarium that many people refer to as the "clean up crew". They recycle the detritus and waste from the larger animals and plants in the setup, eating fallen plant parts, fecal material, and sheds. The plants, in turn, use the waste from the isopods as fertilizer to grow. Isopods also burrow through the soil and aerate it to encourage healthy plant growth. Dwarf whites can be used as the only isopod in a setup for smaller animals, or they can be part of a clean up crew of many layers. Many keepers often use a combo of dwarf white and Porcellionides pruinosus for their setups, or larger, more aggressive species for large animals such as boas and skinks.
Trichorhina tomentosa stripping an elm tree leaf.
Dwarf whites do have one interesting trait that is found in only a few isopod species: they are parthenogenetic. Parthenogenesis is when viable offspring are produced without assistance from a male, the female essentially clones herself in these instances. While it is a fluke with some species, (it is often observed in reptiles that have not been exposed to males), this is the only reproductive method in dwarf whites. A single female can produce 30 or more mancae every month! Due to their productivity, a thriving dwarf white population will prevent the introduction and spread of many detritivore mite species such as wood mites and grain mites by out competing them.
A gravid adult female Trichorhina tomentosa.
The one downside of dwarf whites is that they cannot be housed with a few invertebrate species. With terrestrial animals, like spiders, they pose no harm, and millipedes tolerate them as well. Larger terrestrial roaches carry about their lives unaffected by their small roommates. The animals that have issues are the smaller burrowing species such as roaches in the Therea genus. These roaches spend more of their time burrowed, even in adulthood, and have a bit of a fragile constitution. While the dwarf whites will not physically harm the adults themselves, their presence seems to stress the gravid females significantly. Females will abort ooths long before the ooths are finished developing, and suffer prolapses in the process. Female roaches will die as a result of this, and the population slowly tapers off. Dwarf white are also known to stress some large species of isopod for this reason. Many giant Porcellio from Spain will get upset having to share a setup with the little guys.
To sum up, dwarf whites have many practical uses for reptile keepers. Even those who do not utilize bioactive enclosures may have a need for these prolific tiny isopods.