Two Basic Mechanisms in Insect Wing Folding

Detailed comparison of patterns of folding in insect wings has shown that all those which fold transversely can be referred to two basic mechanisms, each consisting of four panels rotating hingewise about four folding lines, intersecting at a single point. The mechanisms, which have one degree of freedom, are in effect lever systems, whose velocity ratios change non-linearly as they operate. They are designated as `Internal' (INT) and `External' (EXT), and further specified by a plus or minus sign, derived from the convex or concave orientation of the folding lines. There are hence four possible combinations: INT-, INT+, EXT- and EXT+. The two types are kinematically distinct. As INT unfolds, the effector panels move slowly at first and accelerate towards the end of the movement, whereas in EXT the effector panels move fast initially, then decelerate rapidly to their their final position. Furthermore INT operates reversibly, whereas EXT usually needs to be closed elastically, or by an extraneous force. The principles of the two mechanisms are applicable to many other folding structures.