The hearing system of the Ensifera insect group continues to surprise.
Though mammals and insects have been evolving separately for a long time (and of course, they’re a much older group than us), a study in November found that how both gr
oups hear is not all that different. Mammalian hearing generally has three stages: first, sound waves vibrate the eardrum (or tymphanic membrane). These vibrations reach the three tiniest bones in the human body, known as the hammer, anvil and stirrup (collectively called the ossicles). These bones amplify the eardrum’s vibrations which then travel through the fluid-filled cochlea. Hair-like cells in the cochlea turn these waves into electrical impulses for the brain to interpret.
Remarkably, the South American katydid Copiphora gorgonensis also has a structure like our ossicles to convert and amplify vibrations. Their eardrums (unlike us, they have two per ear and hear using their legs) send vibrations to a plate-like structure. This ossicle-alternative amplifies the vibrations and sends them a simplified cochlea. The discovery was completely unexpected – not only is it a great example of convergent evolution, but it also shows sound analysis can be both incredibly small and yet extremely refined.
The second surprise is more recent, and possibly more staggering. Analysis of the New Zealand weta has revealed that they have a hearing component previously thought to be unique to toothed whales such as dolphins.
It was previously known that one of the components of the Ensifera hearing system sits within a fluid-filled channel. This fluid was previously assumed to be hemolyph (the insect equivalent of blood) but is actually a new kind of lipid – a hearing component only toothed whales were thought to have. Further study revealed the lipid-producing organ, now named the olivaris. Though only the weta was studied, the team believe the same component is present in all Ensifera insects.
It’s certainly a surprising result. “We don’t know why animals who are so far apart in evolutionary terms have this similarity, but it opens up the possibility that others may use the same system component,” said James Windmill (University of Strathclyde and co-author).
Photo credit: C. Williams.
Sources on katydids and mammalian hearing:
Sources on wetas and toothed whales:
The paper http://bit.ly/ToI6Dg