The plural form referring to mites and ticks, tiny arachnids that are usually invisible to the naked eye.
From Latin 'acarus', derived from Greek 'akari', possibly from 'akara' (mite), referring to these microscopic creatures.
You have millions of dust mites living on your skin and bedding right now—they're a type of acari—and they've been sharing human habitats since before we even knew they existed because they're so impossibly small.
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