You hear it before you see it. Not because it is buzzing, though you do hear an ever-growing hum from above you. Instead, what stands out the most is the thud every few seconds. You look around you, wondering what this ungraceful thing could possibly be. And then you see it. Moving clumsily through the air, drifting to the left and right, colliding with anything in its way. It’s a Figeater Beetle (Cotinis mutabilis). It eventually exits the scene. Leaving behind one question, why does it fly like a drunk driver?
To answer this, we must look at general beetle (Coleoptera) anatomy. The most distinguishing feature is their elytra. Elytra are the hard, fully sclerotized protective forewings covering the beetle’s back. Usually, beneath these elytra are membranous hindwings, which they use for flight. However, some beetles have modified elytra, for example, Carabidae (ground beetles) have fused elytra, so they can't fly or Staphylinidae (rove beetles) have shortened elytra (NIH). Interestingly, Figeater Beetles are so big it is hard for them to lift there elytra especially in flight, so they don't. This means they have very limited accuracy since they are already forcing their hindwings out from under their elytra to even get into the air (NHM).
Comparing beetles to other insects reveals that elytra are one of their unique traits. Most other insects have two pairs of membranous wings. Though Hemiptera (true bugs) have partially sclerotized forewings and Diptera (flies) only have one pair of wings with their second pair being minimized to halteres, which do still aid them in balance for flight. Since beetles have these heavier, hard protective wings, it makes flying different. They can’t use their elytra to fly, so they must lift them high enough to unfold their hindwings, which are specifically folded beneath. Or if they are like the Figeater Beetle, they keep their elytra down, but instead force their membranous wings out from underneath.
If other insects survive without elytra, where did this adaptation come from? What is it for? It turns out that they likely developed first in ground dwelling beetles who often inhabited woody areas. Having a hard covering not only protected them from scraping their wings as easily, but it eventually also protected them from falling from heights. A study was done on ladybird beetles and it found that when falling from a height, they were most likely to orient themselves to land on some part of their elytra, specifically the costal edge. This was seen to add protection to the beetle and reduce impact from the fall (JIP). This supports the idea that beetle elytra likely evolved as protection. However, elytra are also know to help conserve water, provide thermal insolation, and for beetles that lift their elytra in flight, it provides lift and balance, which again is why Figeater Beetles are clumsy because they don't have this advantage (NIH).
Though elytra can help protect beetles from their environment and predators, many have additionally developed specialized elytra for a variety of different functions. This variety has given them an evolutionary advantage, which is why they are one of the most diverse and dominant groups of organisms on the planet. Some of these special adaptations include for chemical defense, camouflage, subelytral chambers, and so on (NIH).
Because of these elytra specializations, beetles have an additional level of diversification within their order, which has increased their overall fitness. In particular, other insects are not able to fit many of the niches that beetles can, though of course there is a cost. Developing an elytra requires extra sclerotization, which requires more energy, specifically more tyrosine, which is a semi-essential amino acid. But many beetles have formed relationships with symbionts as a solution for this (NIH).
And so next time you see a Figeater Beetle, don't make the generalization that all beetles are bad flyers and remember instead that the elytra that limit their flight also gives them protection and many other benefits.
Sources
https://pmc.ncbi.nlm.nih.gov/articles/PMC9975656/ (NIH)
https://nhm.org/stories/when-fig-beetles-attack (NHM)
https://www.sciencedirect.com/science/article/pii/S0022191024000180 (JIP: Journal of Insect Physiology)
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