Wing coupling

Some four-winged insect orders, such as the Lepidoptera, have developed a wide variety of morphological wing coupling mechanisms in the imago which render these taxa as "functionally dipterous" (effectively two-winged) for efficient insect flight.

[3] The more primitive groups of moth have an enlarged lobe-like area near the basal posterior margin, i.e. at the base of the forewing, called jugum, that folds under the hindwing in flight.

Along with the frenulum, a spine at the base of the forward or costal edge of the hindwing, it forms a coupling mechanism for the front and rear wings of the moth.

In the butterflies[a] and in the Bombycoidea[b], there is no arrangement of frenulum and retinaculum to couple the wings.

Despite the absence of a specific mechanical connection, the wings overlap and operate in phase.

Oiketicus spp. (family Psychidae ). The frenulum can be seen at the top of the rear wing, which hooks onto the retinaculum, seen here as a small brush on the front wing, so that the wings travel together during flight. Magnification: 10x
Examples of frenulo-retinacular wing coupling in male and female moths