frenum vs. frenulum

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

Anatomy, Zoology. a small frenum.

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. Compare jugum ( def 1 ).