Sentence:  Earless ghost swift moths become “invisible” to echolocating bats by forming mating clusters close (less than half a <WSD>meter</WSD>) above vegetation and effectively blending into the clutter of echoes that the bat receives from the leaves and stems around them. 
Meaning of the word marked with <WSD> tags:  The base unit of length in the International System of Units (SI), conceived of as 1/10000000 of the distance from the North Pole to the Equator, and now defined as the distance light will travel in a vacuum in 1/299792458 second.