Why Did Insects Grow So Huge Around 300 Million Years Ago — And What Made Them Shrink Again?

Imagine a dragonfly with a wider wingspan than your shoulders, flying around, buzzing through a swampy forest. That was the world back then, around 300 million years ago, when insects, bugs, and other arthropods were gigantic. It is a period on Earth that occurred even before the arrival and domination of the dinosaurs, called the Carboniferous Period.

However, a few million years later, most of these giant species gradually disappeared, and their modern-day relatives are now significantly smaller. But why exactly did they shrink, and what made them have an enormous size in the first place? And did they really rule the skies during their time? Let’s explore the science and history behind these ancient giants in this article.

Background — Life during the Carboniferous

Approximately 359 to 299 million years ago, the skies on Earth were ruled by giant insects like the Meganeura and an even larger Meganeuropsis. They were aerial apex predators of their habitats, which were usually a swampy forest with massive vegetation and an atmosphere that was very different than that of today.

This now extinct insect, Meganeura, had a wingspan of up to 65 centimeters or more than two feet, while the Meganeuropsis had up to 71 centimeters, with a body length of up to 17 inches or 43 centimeters. To put these sizes into perspective, just imagine a giant insect the size and weight of a modern-day hawk.

Additionally, it is not only the insects and bugs that were gigantic back then. Even other species of arthropods, like the Arthropleura, the largest known terrestrial invertebrate in Earth’s history, were also roaming the planet. It is estimated to be capable of reaching up to 2.6 meters or 8.5 feet in length and weighs about 50 kg or 110 pounds.

Read more: Wikipedia

Why Insects Could Grow So Big

The Terrifying World of Giant Insects: What Was Earth Like During the Carboniferous Period?

There are a few reasons why insects became large during the Carboniferous Period, but one of the main factors is the air they breathe. During this time, it is estimated that there may have been a higher amount of oxygen in the atmosphere, up to 35%, compared to today’s 21%.

This may be because the massive forests and vegetation grew and died, but their remains may have been buried in the swampy environment of the planet. This scenario could have prevented the decomposition of the plant matter and the release of carbon dioxide back to the atmosphere.

Then, as the plants in the large forests continually perform photosynthesis, there may have been a surplus of oxygen in the air, which would have raised the oxygen levels of the entire planet. On top of that, the insects breathe through their tracheae, unlike humans, who use their lungs instead. This meant that they are capable of moving more oxygen into their tissues and bodies when they breathe the air.

Another reason besides the difference in atmosphere is the abundance of plants and food for the insects. The Carboniferous Period was a time on the planet when there were large swampy forests, a great habitat for these insects to grow and prosper.

Moreover, it is also believed that there may have been a lack of predatory species that could hunt these insects in the skies. It is only later, about 150 million years ago, that new aerial predators, like the early species of birds, such as Archaeopteryx, arrived in their habitats.

Combined, all these factors may have played a role in why the insects and other arthropods evolved to be huge in size. But why did they disappear, or why did their relatives become smaller in the modern world?

Read more: National Geographic

What Caused Their Shrinkage and Disappearance

If one of the main reasons why the Carboniferous Period insects were so huge was the oxygen levels in the air, the inverse could also be the reason why they shrank in size. Over time, the Earth’s oxygen levels in its atmosphere dropped or fluctuated, and one of the factors for this is the evolution of Fungi, which are capable of eating plants with lignin.

The massive plant growth that allowed the increase in oxygen during this time was primarily from the evolution of plants with lignin, a polymer that made wood strong and able to grow in height. This lignin was previously indigestible, so along with the swampy environment, it is hard for it to decompose and release carbon dioxide back to the air.

However, when the fungi evolved the ability to eat these plants with lignin, it meant more buried plant matter started to decompose, releasing the stored carbon to the atmosphere. Additionally, they also consumed oxygen, and ultimately, their rise contributed to the decrease in oxygen levels on the planet along with other factors.

The second reason why insects shrank is attributed to the development of agile birds. Previously, these insects were the aerial apex predators of their habitats, but when the new flying creatures appeared around 150 million years ago, the giant size of the insects became less of an evolutionary advantage.

The third reason is environmental and climate change. The swampy forests that were once filled with food sources and natural habitats for giant insects dried up or became fragmented. So, the ecosystems that favored and allowed the existence of the giant insects and arthropods have disappeared.

Then, in smaller forests or open areas, the smaller insects may have had more advantage as they would be better at flying, hunting, or hiding. They could be the ones that survived and reproduced and passed on their genes to the next generation, leading to the gradual decrease in size of the once giant insects.

Read more: News

Why It’s Not Just One Cause

What Caused the Extinction of Giant Insects? | Full Documentary

The oxygen levels or the appearance of the flying vertebrates are both significant factors, but experts generally believe that there may not be just one primary cause for their disappearance, but rather the combination of different reasons is what made them what they are today.

So, in other words, the giant insects faced a lot of challenges that may have favored a smaller size over time. The drop in oxygen levels affected their physiology, the appearance of the birds who became the new dominant flying species influenced their ecological habitats, and the changes in the environment and climate limited their ability to grow and thrive.

Additionally, there may have been many giant insect species whose fossils did not survive the last 300 million years. It could be that they did not all disappear overnight, but instead some of them developed to be smaller over the years because of the stated reasons, and the other giant insect species could have been outcompeted and completely eradicated.

In short, the disappearance of giant insects is gradual and has happened over millions of years, and is caused by a multitude of factors, not just a single event or cause.

Author's Final Thoughts

The story of giant insects from the Carboniferous era shows just how much life is interconnected on this planet. The evolution of different species, whether they are animals, insects, plants, or fungi, could influence the environment and change the rules of survival on Earth.

Understanding this scientific phenomenon helps us comprehend that what we consider natural in nature could just be a moment in time, and in the long history of evolution and adaptation of life.

Read next: Scientists Found Female Frogs Sometimes Play Dead to Avoid Mating with Unwanted Males — And Here’s the Strange Reason Behind It

References & Further Reading

Dudley, R. (1998). Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance. Journal of Experimental Biology. https://journals.biologists.com/jeb/article/201/8/1043/7948/Atmospheric-Oxygen-Giant-Paleozoic-Insects-and-the

Klok, C. J., & Harrison, J. F. (2009). Atmospheric hypoxia limits selection for large body size in insects. PLoS ONE. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0003876

Harrison, J. F., Kaiser, A., & VandenBrooks, J. M. (2010). Atmospheric oxygen level and the evolution of insect body size. Proceedings of the Royal Society B. https://royalsocietypublishing.org/doi/10.1098/rspb.2010.0001

Kaiser, A., et al. (2007). Increase in tracheal investment with beetle size supports hypotheses of oxygen limitation on insect gigantism. Proceedings of the National Academy of Sciences. https://www.pnas.org/doi/10.1073/pnas.0611544104

Clapham, M. E., & Karr, J. A. (2012). Environmental and biotic controls on the size distribution of Late Paleozoic insects. Proceedings of the National Academy of Sciences. https://www.pnas.org/doi/10.1073/pnas.1204026109

Clapham, M. E., & Karr, J. A. (2012). Reply to Dorrington: Oxygen concentration and predator escape abilities are important controls on insect size. Proceedings of the National Academy of Sciences. https://pmc.ncbi.nlm.nih.gov/articles/PMC3528513/