Cow-sized locusts, dragonflies the size of hawks, and cockroaches as big as a baby elephant...
pretty terrifying, don’t you think?
Sci-fi horror films have been imagining such monstrosities to terrify audiences for decades,
but are these nightmares possible?
Is it possible to have giant insects?
To answer this, we need to look at a few basic aspects of insects.
First of all, consider the insect’s skeleton.
Insects,, like other arthropods, such as spiders and scorpions, have their skeleton on the
outside; this is called an exoskeleton.
It is made from a hard, durable and water-resistant substance called chitin.
On the other hand, internal skeletons, like the skeletons of birds, reptiles and mammals,
are on the inside, supporting muscles and cells from within.
An exoskeleton is great for insects and arthropods, since it protects them from the many dangers
of the world.
Muscles directly attach to the exoskeleton and this outer covering provides the perfect
environment for the insect’s delicate organs to function.
The exoskeleton is ideal for small insects, but not so effective for larger ones.
You see, as an insect gets larger, let’s say to the size of a cow or a human, it would
need a significantly thicker exoskeleton to protect its increasing weight.
Consider this: if an insect doubles in size, its surface area increases by four times and
its volume and mass increase by 8 times.
Thus, as an insect gets larger, the surface area actually decreases relative to the volume.
This means that moving with such a large body would be a nightmare.
The giant insect would need more muscle power in order to move, and it would still not have
the same agility as its smaller counterpart.
With a smaller surface area relative to volume, there wouldn’t be enough exoskeleton to
connect with all the necessary muscles and cells.
Without increased muscle power, the legs might not be able to handle the increase in weight
and the insect might have to add more legs to its body, at which point it could no longer
be called an insect.
It would be a new type of animal altogether!
Another factor to think about is how an insect exoskeleton grows.
They don’t develop in the same way internal skeletons form.
Human bones and muscles grow gradually as we increase in size.
Insects, however, molt out of their smaller exoskeleton into their larger, adult exoskeletons.
In the midst of their exoskeleton wardrobe change, they expose their soft and delicate
inner bodies, making them an easy and tasty meal for predators.
That being said, much of this is speculation based on the biological evidence that we have
today.
However, one study calculated the increase of exoskeleton size with the increase in overall
size of an insect and found that while an increase in exoskeleton is a limiting factor
in insect growth, it isn’t the primary factor.
The problem actually appears to be how to produce energy more efficiently, which leads
us to what scientists think is one of the main reasons for insect size—OXYGEN!
Oxygen is the key to our energy production.
All animals that breathe in oxygen require it to produce their energy.
The oxygen levels on Earth today are about 21%, and even the largest insects today are
no more than the size of our palms.
While that is the case in the year 2020, the oxygen levels 300 million years ago were 31
to 35%!
The insects on Earth 300 million years ago were indeed the size of hawks and puppies.
One dragonfly cousin, griffinflies, were as large as hawks.
Scientists are quite confident that the increased oxygen levels allowed insects to get much
bigger than we see today.
Insect size is more dependent on oxygen than that of mammals because of how insects breathe.
Birds, mammals, and other large terrestrial animals breathe through lungs and transport
through blood vessels directly to the cells.
Insects, on the other hand, breathe in through tiny holes lining the sides of their body
called spiracles.
Oxygen taken in through the spiracles makes it through tubes called trachea and is then
diffused to the other cells of the insect.
This mode of breathing limits how much oxygen insects can take in based on how much oxygen
the trachea can distribute throughout the body.
If the insect were to grow to a giant size with limited oxygen, the number of trachea
would have to be much, much larger.
Many studies that have compared juvenile insects to larger adult insects found that adults
have many more trachea.
Another study also found that dragonflies fly better in an oxygen-rich environment!
While that is a strong second explanation, a decrease in available oxygen isn’t the
final reason insects are small today.
When giant insects were roaming the earth, birds and bats hadn’t evolved yet.
Around the time the first birds began roaming the skies, roughly 150 million years ago,
the number and size of giant insects simultaneously began to decrease.
Giant insects took another hit when bats evolved.
Both birds and bats, so the hypothesis goes, might have competed with giant predatory insects
for prey.
Since the birds were more agile and flexible, they were able to out-compete the ancient
giant dragonflies and cockroaches for food.
All of these situations and events may have jointly contributed to the decrease in insect
size.
Despite their small size, insects are very cool.
They have complex and perfect social structures, can lift things that are many times their
own body weight, and boast all sorts of cool colors and unique adaptations.
So what if they can’t grow to the size of cows… all the better for us anyway!