Why are insects so small?

If you’re scared of creepy crawlies, be thankful you didn’t live in the Carboniferous period. Around 300 million years ago, giant insects would roam the planet. The largest being from the extant genus Meganeura which resembled modern day dragonflies. Their wingspan could reach over 2 feet! Back then there were centipedes over 8 feet in length, guitar sized scorpions and ants the size of birds. So why are they so small now? And could they grow back to these lengths?

Simple answer being we still don’t really know. But there are two main theories. The most studied and accepted theory being that the size change was in relation to the amount of oxygen in the air.

Insects don’t have lungs. Instead they rely on small openings called spiracles on their thorax and abdomens to bring in oxygen to their tiny trachea, which then passively transport it to the body cells.

Due to the small size and bottle-necking of an insects trachea, it affects how much oxygen they can take in, and therefore their size. If they were to be any bigger than they are now, they would need a lot more oxygen to survive. But the quantity needed would be too much to fit in their narrow trachea’s.

The quality of air millions of years ago was much different (as you can imagine). It was warmer and wetter. 200-300 million years ago, the amount of oxygen in the atmosphere was around 35%. Allowing insects, like those in the Meganeura genus, to get so big. Today, oxygen in the atmosphere is only about 21%. So basically, insects had to adapt to this reduction in oxygen by becoming smaller. If oxygen levels were to increase again, in theory, insects could become larger.

Insect respiratory system

Insects circulatory system is also different from ours. They have an open system (which is seen throughout a lot of invertebrates) where blood is not transported in blood vessels. Instead it flows freely around the cavities and sucked up back into the circulatory system. The bigger the insect, the harder it would be to move blood around the body. Gravity would draw the blood down making the uptake more difficult. This could also be another reason why insects had to shrink.

Another prominent theory to why insects have become so little is due to the evolution of birds. Despite oxygen levels decreasing, around 150 millions years ago oxygen became more abundant in the air. But insects kept getting smaller. At the same time, the first birds started to evolve from the Theropod dinosaurs.

As the dinosaurs took to the skies and became more efficient at flying, they forced flying insects to be more manoeuvrable. Which drove evolution to favour a smaller body in those species. Much like birds today, early ones were thought to feed on small insects that existed at the time. The larger insects were thought to be predatory and also feasted on the small bugs. This increased competition between birds and large flying insects also put a cap on how big the insects could grow.

So with birds today being so specialised and proficient flyers and the poorer air quality, insects today are forced to remain the tiny creepy crawlies we often accidentally tread on. But why they aren’t any bigger or what the biological cause of controlling body size is still widely unknown.

Today, the largest insects to roam the planet aren’t anywhere near as big as their ancestors. The Titan Beetle (Titanus giganteus) which can be found in the amazon rainforest, can grow up to be 6.6 inches long. It also has powerful mandibles that could chunk off human flesh. And with a wingspan of around 11 inches, The Atlas moth (Attacus atlas) is considered the largest moth in the world!

Titan beetle (Titanus giganteus), one of the largest insects in the world today.

But this also brings in the question of why some animals are so big? The largest living animal alive today is the blue whale (Balaenoptera musculus). Reaching a whopping size of up to 100 feet long. It weighs around 180 tonnes and has a heart the size of a bumper car which only beats around 2 times a minute. But why are the big? And could they also get bigger?

On land there are some big creatures like the mighty Elephant. Being big allows you to scare off predators, have better fighting abilities and also you can store more water and fat. But land mammals are limited by gravity. Bulkiness means large bones and blood vessels to support the weight and mobility. Underwater, gravity isn’t an issue.

According to some scientists, marine mammals need to be a minimum size in order to survive. That minimum size being a thousand times bigger than the smallest land mammal. This is due to the need of heat and food.

Being underwater, the smaller you are, the quicker you lose heat. It’s almost impossible to eat enough food to keep up with the heat loss. So marine mammals have evolved to be a size that allows them eat enough food to provide enough heat to survive. Whales have taken this to the extreme.

Whales first appeared 50 million years ago, but it took another 10 million years for them to become completely aquatic. Some species evolved baleen which allowed them to take huge gulps of water and strain it for prey (e.g krill). This proved to be the most efficient feeding strategy compared to how toothed whales feed. They could take in massive amounts of calories in comparison to how much they burned while foraging, which enabled them to get bigger, which in turn makes them more efficient feeders.

Whether or not the blue whale or insects could grow in the future is unknown. What we do know is that both these gigantic and diminutive beasts are now facing extinction. Blue whales are now classed as endangered, and according to 2019 study, 40% of insect species are heading to extinction, with insect mass falling 2.5% every year.

Big or small, human or not, we all live on the same planet. We can’t lose the amazing biodiversity we have the privilege to see today. Everyone and anyone can do their bit to save the planet. Then we might be able to see what evolution has in store for all of the worlds creatures. Who knows, giant centipedes might make a comeback (I hope that hasn’t scared people into not saving the planet lol)!

Now enjoy some pictures of some prehistoric giant animals! (* WARNING, GIANT SPIDER INCLUDED*)

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