Part of the show Science of Sight, Eye Diseases and Animal Vision
A study on beetles has revealed why there are no longer 3 foot-long dragon flies buzzing around in your garden (as there were 300 million years ago) - and it's all down to oxygen. Alexander Kaiser, from Arizona's Midwestern University, together with his colleagues x-rayed four different beetle species of increasing size including the 3mm long Triboleum castaneum beetle and the much larger 3.5 cm Eleodes obscura. The aim was to measure the relative sizes of the tubes, called tracheoles, which they use to draw air into their bodies. These tracheae run down the sides of an insect's body, and the animal moves air in and out by pumping its abdomen. But the x-rays showed that, with enlarging body size, the tracheae became disproportionally big, and take up 20% more space than the increase in the insect's body size would predict. This becomes a problem at the site where the body and legs meet. The opening connecting these two structures can only become so big, limiting the size of the trachea that can pass through and therefore the size of the beetle. The team calculated, on the basis of their measurements, that it shouldn't be possible for a beetle to exceed about 15cm in length. And indeed the largest beetle species currently on Earth is the appropriately-named Titanic longhorn from South America, which grows 15-17cm. But, this is based on present-day oxygen concentrations of about 21%. 300 million years ago oxygen levels were much higher - closer to 35% - and as a result insects didn't need such big trachea and huge insects, including dragonflies with a three and a half foot wingspan were common.