Wiktionary
n. The smallest of the faunal size divisions, including mainly microorganisms but also sometimes applied to the tiniest species of animal groups such as ticks, insects, etc.
Wikipedia
Microfauna ( Ancient Greek mikros "small" + New Latin fauna "animal") refers to microscopic organisms that exhibit animal-like qualities. Microfauna are represented in the animal kingdom (e.g., nematodes, small arthropods) and the protist kingdom (i.e., protozoans). This is in contrast to microflora which, together with microfauna, make up the microzoa.
Usage examples of "microfauna".
Describes unique design he deduces in himself: special intestinal microfauna plus superefficient body chemistry extracts enough energy to live on from blood.
Illa knew, spread for acres, holding the soil and secreting microfauna to defend against native parasites and rework the local minerals into compatible nutrients.
If a mat was smuggled and not passed through sterilization, microfauna came in the reeds.
Originally, I'd planned to specialize in the ecopoiesis -- and to study much closer to home -- but in the end I'd had to accept the nearest thing on offer, geographically and intellectually: working with Barat, a Firmlander biologist whose real interest was native microfauna.