Saturday, July 28, 2012

Insect headgear in the headlines

In May of last year, treehoppers (Membracidae) were being talked about because of what their helmets (1st image) show about their evolution. These varied structures - some of which resemble bird droppings, dead leaves, thorns, even in one species an ant in attack position - have been shown to originate not as simple outgrowths of the cuticle, but dorsal appendages attached to each side of the thorax by an articulation, with muscles and a flexible membrane that allow it to move. Examined under the electronic microscope and confirmed genetically, the helmet is in fact a 3rd pair of profoundly modified wings. It is the 1st example in 250 million years of insect evolution that a new set of wings has appeared, rather than an existing set transforming or disappearing. Since its debut around 40 million years ago, the helmet has totally freed itself of the structural constraints linked to flight. The team from the Institut de Biologie du Développement de Marseille-Luminy (CNRS) led by Nicolas Gompel and Benjamin Prud'homme concludes, "In short, it's a wing that's no longer a wing. Freed of its flight function in this group of insects, this wing has been able to diversify its shape and texture without moderation."

This month, the Japanese rhinoceros beetle (Allomyrina dichotoma) is making news because of its headgear (2nd image), which can grow to as much as 2/3 of its body length. Scientists have discovered that not only is the size of the forked horn useful for flipping rival males off a branch (see video here*), it is an unfailing indicator of his health and prowess. The horn is 8 times more sensitive to insulin and related molecules called insulin-like growth factors (IGFs) than any other body part, so well-fed beetles may have larger wings and bodies than poorly-fed ones, but they have much larger horns. A study by entomologist Doug Emlen of the University of Montana confirmed that the loss of insulin signals - triggered artificially in his experiments, but naturally by lack of nutrition - resulted in their horns growing 16% smaller. Dr. Emlen has shown that the reason the beetles can't grow flashy horns is not that they would need their strength to bear the heavy burden, but that the size of those ornaments is tied to their nutrition-dependent insulin levels and that can't be cheated. "They can’t fake their way to showiness."

*While researching this post, I learned that insect fighting is a popular spectator sport in Japan, China,  and other Asian countries!
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