Frenulum (butterfly)
The frenulum , in conjunction with the retinaculum , enables most butterflies to couple the two pairs of wings together during flight so that they can be moved as a unit. In the males it consists of a single strong bristle, in the females there are usually two to nine adjacent bristles, only in some species, such as the glass winged moths ( Sesiidae), real moths (Tineidae) or some owl butterflies (Noctuidae) also have them Females only have one bristle. The frenulum arises from the costal plate of the rear wing, a structure that is located at the base of the wing. The frenulum is directed towards the retinaculum, which is located on the underside of the fore wing.
Some primitive butterfly species and also the caddis flies have a number of costal bristles on their hind wings. Since they at separate locations at the base of Costa spring and not at a point of the costal plate, they are distinguished from frenulum.
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- Malcolm J. Scoble: The Lepidoptera: Form, Function and Diversity . Oxford University Press, Oxford 1995, ISBN 0-19-854952-0 (English).