Ancient Fossil Katydids Hint At The Origin Of Insect Ears

A katydid forelimb. The ears are the small white
dots. They are less than a millimetre in diameter

The Green River Formation in Wyoming is most famous for its fossil fish due to the degree of preservation and the striking contrast between the dark brown remains and the pale limestone. A whole range of different organisms have been found from stingrays, crocodiles to birds. 48 million years ago, the deserts of Wyoming were very different: instead of rock and sand, they were covered in vast lakes fringed by rainforests and mountains.

The most common finds in all three habitats are insects: thousands are recovered every year. These fossils are helping to solve a palaeontological mystery: how and when their ears evolved. A difficulty lies in the fact that insect ears appear on different parts of the body across the various insect groups. For example the grasshoppers have ears on their abdomen while those of the lacewings are on the front legs.

This would suggest that insects gained ears at different times throughout their history. Yet a general rule of evolution is that large organs, in this case the ears, only evolve once and if they are lost, do not reappear. This is the polar opposite to other features such as fur colour which can come and go as much as demand requires. The single time approach theory attempts to solve this. It links the evolution of insect ears to the origin of bats, indeed the earliest fossils have been found around the Green River Formation.

Bats use supersonic frequencies to communicate. Therefore insects would have evolved sensitive ears to detect these frequencies. Their enhanced senses would have acted like a biological radar, giving them forewarning of the oncoming mammalian predators. A team, led by Dr Roy Plotnick from the University of Illinois, analysed 500 drawers of specimens. The items in question were a mix of katydids and grasshoppers.

They studied the ears, situated on the front legs, and found that they were incredibly advanced, with a fully formed membrane for detecting sound waves. Such organs must have taken millions of years to evolve, putting the date of origin long before bats appeared in the fossil record. As the ear design of insects is quite uniform across the groups, it would suggest that there was a single origin. Yet the sheer number of different species indicates that the evolutionary process may have been accelerated by the evolution of other animal groups, in this case, bats.