The "oxygen constrain hypothesis" has failed to explain why we no longer see large dragonflies the size of dragonflies in the late Palaeozoic era, which were once large, with a wingspan of over 70 centimeters and weighed 100 grams. The theory suggests that the larger an insect grows, the further the oxygen must travel to reach the deepest tissues, leading to a decrease in atmospheric oxygen levels. Edward Snelling, a professor of veterinary science at the University of Pretoria, believes this theory is wrong. Unlike insects with no central pair of lungs and a closed circulatory system, insects breathe through internalized tubing called the tracheal system, which allows them to pump air into larger tubes that are embedded deep within their tissues. The problem with diffusion, which is notoriously slow, requires larger or more numerous tracheoles to maintain the supply of oxygen.