Final answer:
Invertebrate parasites such as helminths are categorized differently from 'true' microbes because they have complex organ systems and are multicellular, unlike bacteria and viruses, which are the primary constituents of the 'true' microbes category.
Step-by-step explanation:
Some microscopic organisms, such as mites and worms, are categorized as invertebrate parasites instead of being categorized as 'true' microbes because invertebrate animals have complex organ systems not found in 'true' microbes. 'True' microbes typically refer to bacteria and viruses, which do not contain complex organ systems. In contrast, invertebrate parasites, such as helminths, may have organ systems for reproduction, digestion, movement, and other functions that go beyond the cellular complexity of bacteria and viruses.
Parasitic helminths are included within the field of microbiology due to their microscopic eggs and larvae, but adult forms of some helminths are visible to the eye. As such, helminths are multicellular and are not technically microorganisms; however, they are studied by microbiologists due to the relevance of their microscopic stages in disease transmission and identification. On the other hand, viruses are acellular microorganisms and have a different biological structure, lacking the complex cell structures of eukaryotic organisms.