Evolving Ears: A Bug’s Guide
A katydid, proudly displaying the front legs in which it houses its ears (Image Credit: Charlie Woodrow, CC BY 2.0)
Insects are famously one of the most diverse groups of organisms, with over one million species discovered, adapted to nearly every niche on the planet. This diversity has allowed for the evolution of an incredible mix of shapes and sizes, behaviours, and other features. Perhaps the most frequently re-occurring of these traits are their ears, with current estimates stating they have evolved up to 20 times independently, on almost every imaginable part of the body. So just what makes it so easy for insects to evolve ears? And why should we study them?
In all of nature we see examples of animals finding their own solutions to the same problems, from fins in sharks and whales, to wings in birds and bats. Similar traits evolving independently in different types of species is known as convergent evolution. In most cases, convergent traits appear to have evolved only a couple of times, and later diversified between species. In the odd case of insect hearing however, it appears finding a novel way to evolve ears is not so difficult!
An insect’s primary use of ears is the same as any animal – to detect mates and rivals, and to listen out for predators. Unlike the ears of vertebrates however, insects have not committed to a single solution to the problem of hearing, and as a result we see a great variety of independently evolved ears, in many locations on the body.
The poem neatly summarises the story of why insects evolved complex hearing organs, which was originally believed to coincide with the appearance of ultrasonic echolocating bats 60 million years ago. However, for many insect groups, the origin of ears is still up for debate. Like many, Pye was impressed by the diversity of these ears, and in his extended sequel to the poem (Read Here) hinted that we have only started to uncover the world of insect hearing.
So just what makes insects so good at evolving ears? And why do they appear in so many places on the body?
The answer to this comes down to several features of insect anatomy, but perhaps most surprisingly, to the role of their respiratory system. Unlike vertebrates, insects do not circulate oxygen through their body via the blood, and do not possess lungs. Instead, they possess a series of tubes called tracheae, which are present throughout the body. These tracheae, and their smaller branching tracheoles, provide a passage for air directly into the tissues for gas exchange, via external openings termed spiracles. Check out the figure below for a simplified diagram.
But how does the respiratory system permit the evolution of insect ears?
Well, for an ear to function, it needs some element of a sound to trigger a response in the insect’s brain. The receptors responsible for the first step of this conversion are called mechanoreceptors. Mechanoreceptors respond to physical stimuli such as stretching, compression, or most importantly in the evolution of ears – vibration. They’re found almost everywhere on the body of an insect. This means insects possess a body covered in receptors with the potential for sensing vibrations, and a series of air-filled tubes that allow the transfer of vibrations. It’s the perfect set-up for ear evolution, and the fact that both receptors and tracheae occur all over an insect’s body mean an ear can evolve almost anywhere.
Then you just need enough time and the right evolutionary pressures for part of the tracheal system becomes associated with a subset of these receptors, forming an efficient system for the passage and detection of sound. Voila, you have an ear!
In these types of ears, the thinning of the body wall between the outside of the animal and the trachea forms a tympanum (eardrum), which allows the organ to become extremely sensitive to even the quietest of sounds. This type of ear is found in crickets, katydids, mantids, moths, and many other insects.
However this is not the only mechanism of sound detection. Other insects use highly sensitive antennae to convert the speed (or velocity) component of sound into a neural response, using a series of receptors at the base of the antennae that respond to motion. These ears are known as the flagellar, or antennal types, and are best known in flies and mosquitoes.
Both ear types are diverse in their sensitivities and frequency ranges, permitting the detection of mates, rivals, and predators in their own ways.
In many insects, a drive to detect the ultrasonic echolocation of bats has also allowed the expanding of their communication system into the ultrasonic range, with some katydids (relatives of the crickets and grasshoppers) for example, singing at over 150 kHz (we are lucky if we can hear above 18 kHz!).
Katydids have the tympanum type ear, with two tympana located on the top of each front leg, but also possess an extra tracheal sound input behind their front legs (via a respiratory spiracle as we mentioned earlier) that allows sound to reach the tympanum on the inside too! The sound they hear is a combination between the inner and outer sound inputs. Their ear has become a favoured model by those interested in convergent evolution due to the functional similarities of their inner ears with our own.
So why should we study them?
It has not been long since my academic journey into the world of insect hearing started (and oh boy, what a time for it), but soon the most common question from friends and family becomes clear; why study insect hearing?
Time and time again, nature has been able to overcome complex challenges with innovative and simple solutions, and few groups of organisms exemplify this ability better than insects. Not only have they evolved simple solutions to hearing, but they have been able to do so while restricted to miniature scales, making their ears wonderful models for bioinspired technologies such as acoustic sensors and small-scale robotics. Their extreme sensitivities, ultrasonic capabilities, and (in some cases) human similarities, are making them increasingly favoured models, and as we continue to explore their diversity, we will surely uncover new independent paths they have taken to evolve.
Charlie Woodrow is a PhD student at the University of Lincoln, UK. He is interested in the evolution of animal communication and ecology, and is currently researching the morphological and functional variation in katydid ears. Follow him on Twitter @CharlieZoology.