The Collaborative International Dictionary of English v.0.48:

Prescutum \Pre*scu"tum\, n.; pl. Prescuta. [NL. See Pr[ae]-,
   and Scutum.] (Zool.)
   The first of the four pieces composing the dorsal part, or
   tergum, of a thoracic segment of an insect. It is usually
   small and inconspicuous.
   [1913 Webster]