the choreography of wing folding

    Generally speaking, insects have two sets of wings. Front wings and hind wings. As different insect groups evolved there have been many modifications to this configuration. From a bioinspiration perspective, it is interesting to consider how many possibilities have been explored through the process of evolution. For example, dragonfly wings are kept in a constant position perpendicular to the body at all times. 

Dragonfly - Image credit: Luc Viatour

     

    Other insects, such as cockroaches, position the wings flat over the back of their body when not in use. Ants, bees, and wasps have hook-like structures (called hamuli) on the front side of the hind wing that fit in a groove on the back side of the front wing linking the two wings during flight. 

Wasp and close up of the junction between the front and hind wings showing the hook-like hamuli (Image credit)

    Beetles have evolved a different wing configuration. The front wings have evolved to be wing covers for the hind wings – they are not used for flight at all. The front wings of beetles, also known as elytra (singular = elytron), are hardened structures that lift up and out of the way when the beetle takes flight. The hind wings are tucked up and out of the way underneath the elytra, like a neatly folded shirt packed in your suit case. The wings, however, are spring loaded so they are ready for action when the elytra are lifted up.

June beetle (Phyllophaga sp.) getting ready for flight - elytra lifted up with hind wings unfolding (Image credit: Bev Wigney)

    Now there is a useful mechanism. Compact structures that can go from being neatly folded, out of the way under a protective covering, to springing into position to propel an insect in flight. Imagine flying down the street for coffee with wings that self-deployed from your backpack. Okay, probably not the current goal for beetle wing bioinspiration - but think of the possibilities. A group of Japanese scientists has already started. Using microcomputed tomography in combination with synthetically made translucent elytra, Saito et al. (2017) studied how ladybird beetles fold their wings up under their elytra. I could go through the play-by-play but I highly recommend you watch the video. The folding action is aided by movements of the abdomen in combination with supporting structures that create friction. The idea is that different aspects of the wing, such as the wing veins, are shaped in a particular way that allows the wing to fold while also spring loading it for deployment. Maybe some of you saw this segment on 60 minutes – one of many ways engineers can draw inspiration from insect folding and deployment mechanisms.

    Of course, Saito and colleagues were not the first to notice the incredible wing folding mechanisms of beetles. One of the earliest pioneers of wing folding in beetles was William Trowbridge Merrifield Forbes (1885 – 1968). W.T.M. Forbes published many papers on beetles and butterflies. In 1924, he published a paper titled “How a beetle folds its wings” where he provided detailed descriptions of wing folding mechanisms based on dried specimens he started collecting as a child. In 1926, Forbes followed up with “The wing folding patterns of the Coleoptera” (Coleoptera = the scientific name for beetles), which was a more extensive treatise using specimens from the Cornell University Insect Collection. In this paper, he described a ‘fundamental plan’ such that “The folding patterns of practically all beetles prove to be derivable by relatively simple modifications from a single fundamental plan…” (p. 42). Inferring this plan from the specimens available reveals the flexible and ingenious mind of an evolutionary morphologist. He went on to describe this plan, and then remarked on the deviations exhibited by all the different beetle groups, of which there are many. The paper was 27 pages long and the last three words were: (To be continued).

Figure 1 from Forbes (1926) - Journal of the New York Entomological Society 34, pgs. 42-68). PDF courtesy of the Biodiversity Heritage Library.

    Forbes’ publications also included a bit of scientific sleuthing. Unlike my scientific sleuthing, which typically goes back to the early 1900’s (although last time it went back to the early 1700’s), Forbes took an investigation of a silkworm moth back to Pliny the Elder (AD 77) and Aristotle (BC 384 – 322). One comment I found particularly interesting was: “Pliny is of course beyond saving by any mere definition”.

    I would love nothing more than to trade in my car for a pair of self-deployable, beetle inspired wings to meet up with Pliny the Elder.