Written by Joe Ballenger

I read a few things on the size of insects and the decrease of size from there ancestors to now are related to the oxygen levels from then to now.

I was wondering if anyone has raised a few generations in an oxygen enriched environment to see if they grew to larger sizes quickly in the favourable atmosphere or if there evolution was fixed to there new size?

Pictured: Life-size replica of Arthropleura, the largest land-dwelling arthropod which ever existed. Image credit: James St. John. License info: CC-BY-2.0

Oh, I love this question. There’s a lot of cool stuff here.

In the early day’s of Earth’s history, arthropods were huge.

The largest land-dwelling invertebrate in history was the millipede Arthropleura, which reached sizes of nearly 7 feet. It was a millipede much bigger than our current millipede expert, Derek Hennen. There were dragonfly-like insects the size of hawks, and grasshopper-like insects the size of small dogs. Any entomologist would have loved to be around those days.

We don’t see land-dwelling arthropods that big nowadays. The largest land-dwelling arthropod alive today is the coconut crab, Birgus latro. It “only” grows to about three feet long, and actually spends most of its life in water.

So why did insects get that big back in the day?

High-Oxygen Breeding

Drosophila flies reared under high (40 kPa), normal (21 kPa), and low (10 kPa) oxygen concentrations and selected for large size for 11 generations. Flies reared at high oxygen concentrations are bigger, but not so big that the size isn’t explained by pre-existing genetic factors. They also return to normal size after being returned to normal oxygen conditions. Image credit: Klok et. al, 2009

One way to investigate this is by raising insects in high oxygen environments to see if they get bigger, but this approach hasn’t yielded many consistent results. Insects are an incredibly diverse group of animals, and each group will encounter vastly different habitats. Because they encounter vastly different habitats, many groups handle oxygen deprivation very differently.

A good example which demonstrates this complexity are vinegar flies, which live in rotting fruit and frequently encounter low oxygen levels. If you rear Drosophila in a high oxygen environment for a few generations, you get bigger adults after awhile. However, these changes are not passed on to the next generations after you start rearing them in standard atmospheric conditions. They’ve adapted to the low oxygen environments of rotting organic matter, and the increase in size is most likely reflects the fact they already live in suboptimal conditions.

However, that being said, you still do get some interesting effects. Namely, the flies reared in high oxygen environments have slightly smaller breathing structures…and that’s actually a really important observation.

Oxygen played a big role

Early in this blog’s history, we talked about how insects breathe. If you haven’t read that post, it might be a good idea to do so because a lot of information in that post will come into play in this post. In fact, we even mentioned that there were other limitations on the body size of insects in that original post.

Fruit fly larva, with trachea generously highlighted using microscopic techniques. Image credit: ZEISS Microscopy. License info: CC-BY-ND-NC 2.0

Very briefly, insects breathe through a series of tubes called trachea. These trachea deliver oxygen directly to the tissues of insects, and how much the insects invest in these trachea depends on how much oxygen is available to the beetle’s tissues. The bigger the beetle, the longer that oxygen has to travel, and the more volume the trachea will take up. If the trachea take up too much volume in the body, there’s no room for other tissues like muscles and stomachs, and the beetles need these tissues to live.

Differing size of beetle trachea from Kaiser et. al 2007. The outside of the leg is highlighted in blue, and the yellow is highlighted in yellow. Trachea size increases dramatically as the beetle size increases, but it’s far more dramatic than any size increase seen in rearing experiments.

Bigger beetles generally devote more space to trachea. A very influential paper, Kaiser et. al 2007, showed that small Darkling beetles (Flour beetles) devote about 2% of the leg to trachea. Larger darkling beetles (Elodes) devote about 20% of the leg to trachea. Kaiser’s team estimated that beetles need at least 10% of the leg filled with other tissues to function, however some beetles have as much as 40% of the leg taken up by trachea.

I think Kaiser’s team’s observations about the trachea are important, because they demonstrate that the largest insects do have some issues with space usage. The bigger an insect gets, the more space it has to devote to breathing, and there’s only so much space the insect can devote to obtaining that particular resource.

However, I also think there are some things which don’t add up.

Skepticism on Oxygen

There’s some geochemical evidence which casts doubt on the idea that oxygen is responsible for big bugs; there’s some doubt over whether oxygen levels were high enough to fully account for giant size. I think a much simpler counterpoint is to look at actual fossils and compare their size with insects which exist today.

Some of these giants are things like Griffenflies that had two-foot wingspans, but fairly tiny bodies. The largest flying insects alive today (the Goliath Birdwing and the White Witch) have wingspans that are about one foot. There are also stick insects which can reach nearly two feet in length. These are all huge insects, and while they don’t approach the size of some of our early giants, but they’re also not that much smaller. There are a lot of really big bugs that are still around in today’s atmosphere.

I think another issue with treating oxygen as the sole requirement for giant insects comes from the fossil record itself. Most insects which lived alongside these giants weren’t huge. In fact, most insect fossils were about the same size of the insects we see today. So while oxygen availability certainly played a central role, I’m not sure it was the only reason bugs got big back in the day.

Size Matters, If You’re a Bug

There’s a lot of reasons animals reach immense sizes, and being large has its advantages. Ladybird beetles, for example, tend to be big enough to attack their prey. This is something seen in moths, as well. Sphinx moths with larger tongues also happen to have larger bodies, and these moths can feed on a wider range of flowers. In both cases, there’s an evolutionary advantage to being big…and there’s no reason to think this wouldn’t apply to at least some of the prehistoric big bugs.

Another hypothesis is that certain groups of animals tend to be large. Odonates, the closest living relatives to the Griffenflies, are a good example which we can use to demonstrate that. The largest damselfly has a 7.5 inch wingspan; the smallest dragonfly has a wingspan of a little under an inch. Wasps, on the other hand, range from microscopic to about a 3-inch bodylength. Although they don’t compare in terms of size to beetles, your average dragonfly tends to be much larger than your average wasp.

This combination of data tells us something about one specific group of giant insects. The levels of oxygen probably contributed to the gigantic size of Odonates millions of years ago, but there was also an advantage to being that big. Odanates are predators which overpower their prey, and larger animals can take down larger prey. The ability to overpower a larger range of animals may have, in addition to higher oxygen levels, contributed to their size.

This equation will be different for every group of insects, because every group of insects has a different array of tactics they use to survive their environment. The factors which allowed the Griffenflies to grow to gargantuan sizes will be different than the factors which allowed Arthropleura to grow to gargantuan sizes.

Oxygen availability is only one factor which determines size…and that idea tends to get lost in many discussions of this topic.

Works Cited