Eight-legged worlds smaller than a millimetre, not one clade
What we call mites spans two huge arachnid orders—Acariformes and Parasitiformes—that most recent genetics no longer treat as each other’s closest kin. Most adults are under a millimetre, easy to miss in soil, water, plant galls, or as parasites from Varroa on bees to human scabies.
The old subclass Acari bundled ticks and mites together, but polyphyly is now the working suspicion for many researchers, even as a few phylogenomic papers still recover monophyly and blame sampling artefacts when they do not. Size usually sits between 250 and 750 micrometres; some adults are only about 100 micrometres. Bodies fuse prosoma and opisthosoma until almost no external segments remain. Up front sits a retractable gnathosoma—chelicerae, pedipalps, and mouth—not a true head with brain or eyes.
Fossils are scarce. Acariform mites appear in Scotland’s early Devonian Rhynie Chert around 410 million years ago; Parasitiformes show up in mid-Cretaceous amber near 100 million years ago. Today roughly 48,200 species are named, with guesses of a million or more still undescribed. Oribatids shred litter and fungi without becoming parasites; water mites and halacarids take fresh and salt water; dust mites descend from nest-parasite ancestors that once fed on living skin and keratin.
Relative muscle power and speed break intuition: tropical Archegozetes longisetosus, weighing about 100 micrograms, can lift up to 1,182 times its mass. Length-for-length, Paratarsotomus macropalpis ranks as Earth’s fastest animal. Reproduction is usually indirect via spermatophores; eggs may hatch after up to six weeks and pass through as many as six immature stages. Most species never trouble people, yet a few drive allergies or ferry disease—and soil communities rebound unevenly after farming, with oribatids taking years while other mite groups return in months.
Source: Mite
