Sentence:  Earless ghost swift moths become “invisible” to echolocating bats by forming mating clusters close...above vegetation and effectively blending into the clutter of echoes that the bat receives from the leaves and stems around them. Many insects probably use this strategy, which is a close analogy to crypsis in the visible world — camouflage and other methods for blending into one's visual <WSD>background</WSD>. 
Meaning of the word marked with <WSD> tags:  A part of the picture that depicts scenery to the rear or behind the main subject; context.