frenum vs. frenulum

noun

Anatomy, Zoology.

plural

frena 
  1. a fold of membrane that checks or restrains the motion of a part, such as the fold on the underside of the tongue.

noun

plural

frenula 
  1. Anatomy, Zoology.,  a small frenum.

  2. Entomology.,  a strong spine or group of bristles on the hindwing of many butterflies and moths, projecting beneath the forewing and serving to hold the two wings together in flight.