Evidence of evolution: The shrinking of insect body size
Insects used to be enormous. Go back in time 300 million years and we would see dragonflies with 28-inch wingspans. The reason, of course, for these ridiculous sizes had to do with the oxygen level in the atmosphere. Whereas we enjoy about 21% of the air we breathe today as oxygen, it was once closer to 30%. Looking through the fossil record will bear out the case: As oxygen levels fluctuated up and down, so did insect body mass – and it did so in concert; as one went up, so did the other, and as one went down, so did the other. That is, until about 150 million years ago. At that point we see oxygen levels rise, but insects actually became smaller. Here’s a likely reason for that change:
Maximum insect size decreased even as atmospheric pO2 rose in the Early Cretaceous following the evolution and radiation of early birds, particularly as birds acquired adaptations that allowed more agile flight.
No one wants to be the big target.
What we have here is a tidbit of evidence for evolution. We see that there is a strong link between oxygen levels and insect body mass. We know this makes sense because insects today require strong levels of O2 in order to work efficiently (this, in addition to simple wind, is one reason insect bites decrease as one increases in elevation, such as during a hike). In other words, we have that part pretty well figured out. However, there is a kink in the idea. Insects stopped following their usual trend at a particular point. Fortunately, we know that this point was when birds started to evolve. As evolutionary theory predicts in its most basic form, increased predation resulted in changes in prey. And as further evidence:
A further decrease in maximum size during the Cenozoic may relate to the evolution of bats, the Cretaceous mass extinction, or further specialization of flying birds.
New selection pressure brought about a massive change in the direction of an entire class of animals, and it probably did so several times. Just as the asteroid that hit Earth 65 million years ago made it so the dinosaurs were no longer the most significant constraint on mammal size, the evolution of birds made it so oxygen levels were no longer the most vital force behind total insect body mass.
Filed under: Evidence, Evolution | Tagged: Evolution, insects, Jered A. Karr, Matthew E. Clapham, PNAS Proceedings of the National Academy of Sciences of the United States of America Skip to main page content Info for Authors Editorial Board About Subscribe Advertise Contact Feedback Site Map Mass |
For the Sake of Science 
My understanding was that inefficient insect O2 absorption methods, book lungs for example, were a major determiner of size and survivability. As you say, they are dependent on high O2 levels for survival, not because they need all that oxygen, but because it’s so hard for them to make use of it.
Interesting about the birds though. That smooths out the increasing O2 level, with the decrease in insect size. Really though, the rise of mammals had to have had a significant effect. We know that insects were, and are, a major source of nutrition for many mammals.
I was attempting to not just steal the phrasing from the paper I cited. That may have led to some confusion. It’s not that insects are inefficient at O2 absorption but rather that they have high metabolic demands on the stuff.
As for the rise of mammals, it may have had a significant effect. I’m not sure. This only addresses up to the point of the rise of bats, which dates to a little over 50 million years ago. That is, they are only addressing a specific mammal rather than mammals in general. Moreover, they aren’t going deep into the rise of mammals.
Inefficient isn’t really the right word for what i was trying to say I think.
I didn’t notice the hyperlink either, I’ll have a look.