Date: 5 Jun 2026
Topic: Science
Codebreakers are a fun way to build numeracy skills.
A long time ago (around 300 million years ago), the percentage of oxygen in Earth's atmosphere was much higher than it is today. In the modern day, oxygen makes up about 21% of the atmosphere at ground level, but during the Carboniferous Period it may have reached around 35%.
During this time, plants evolved a tough structural compound called lignin, which allowed them to grow much larger and stronger than before. When these giant plants and trees died, the fungi and bacteria that normally decompose dead matter had not yet fully evolved the enzymes needed to efficiently break down lignin. As a result, large amounts of plant material accumulated instead of rotting away. Because decomposition consumes oxygen and releases carbon dioxide, this led to oxygen levels gradually rising over millions of years.
Now, dragonflies belong to a group of animals known as arthropods, which includes insects, spiders, centipedes, and other creepy-crawlies. Arthropods have exoskeletons, which are hard external shells that support and protect their bodies. Insects do not have lungs. Instead, they breathe through tiny openings called spiracles, which connect to a network of tubes called tracheae that deliver oxygen directly to their tissues.
Oxygen diffuses through these tracheae and into the body's cells. Because this system relies on diffusion rather than lungs and blood carrying oxygen, higher atmospheric oxygen levels allowed insects to grow much larger than they can today.
Some of the most impressive examples of these giant arthropods were millipede relatives such as Arthropleura, which could grow to more than 2.5 metres in length, making them among the largest land invertebrates ever to exist.