the spine, simple in males, compound in females, arising from the base of the hind wings in many Lepidoptera (moths, butterflies) and projecting beneath the forewing. The frenulum is comprised of one or a group of fused setae (bristles) and it's function is to unite the wings in flight; - in Cicada, the frenulum is the triangular lateral piece on the mesonotum (middle back) which connects with the trochlea (thickened base of the hind wings). View image
One or more bristles situated near the base of the costal margin of the hindwing in most Lepidoptera. The frenular bristles are held by a retinaculum located on the ventral surface of the forewing. See also retinaculum, wing coupling mechanisms.