Lawrence Woodworth Jr & Wikimedia Commons
The insect has an air-filled cavity in its thorax that is sided by two large plates. It alternates the contraction of the two plates at a rapid pace to produce two distinct clicking patterns against the underlying ribs. both of these patterns resonate within the hollow cavity and amplify each other to produce the dreaded cicada hum.
The US Navy has taken an interest in the insect in an effort to use this mating call as a sound producing mechanism for a new active sonar system. As it is now, these sonar systems are too large and complicated to be incorporated into most of the navy's vessels, so passive systems are used instead, which can only detect moving objects louder (and usually bigger) than the vessel collecting sound. The active system would project sound and collect it as it bounces off of objects, regardless of their sizes, and should be more compact.
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