Most insects breath passively through their spiracles

Fig 1. Ontogenetic allometry in humans (Moore, 1983).

Allometry, also called biological scaling, is defined as the change in organisms in relation to proportional changes in body size. It is, so to speak, a rule of proportionality.

There are different meanings for this term: the ontogenetic one is the most famous definition, i.e. changes in the proportions of different body parts during the development of an organism. 

A human baby has a bigger head proportionally to the size of his body than an adult due to the different growth rate of each body part (is important to add that each body part will always keep a link of proportionality with the other ones, and this link is genetically determined).

In our case of study, the meaning of the term allometry is one related with a more anatomical point of view. 

Insects and the rest of arthropods are covered by a more o less hardened exoskeleton known as cuticle, which is periodically replaced by molting processes (or ecdysis).

This cuticle implies a lot of constrains on growth for these organisms due to its “heaviness”.

So, if an ant grow to be as big as an elephant, the weight of its cuticle would be out of proportion. 

This fact acquires more relevance if we take into account that arthropods are invertebrate organisms, so they don’t have an internal structure that makes them internally solid; thus, the most feasible scenario would be that their legs couldn’t hold their body weight, so they will collapse.

In order to hold the huge weight of its cuticle, its legs must be probably as large as the ones of an elephant (or even bigger!). 

Fig 2 and 3. Relations between the growth of the corporal volume and the growth of locomotor appendices (Images from the subject “Tree of Life”, Master’s degree in Biodiversity, University of Barcelona).

So, if its body grows, their legs must be larger to hold the entire weight of its body (each body part has an specific growth rate: allometry).

 
It would be the same with wings: if a dragonfly became as big as a golden eagle, it must have wings of several meters.

But there is a problem: body parts don’t grow to the infinite because they have anatomic and genetic constrains which modulate the size of each organ proportionally to the rest of the organism in an harmonic way. 

This drives us to talk about the existence of trade-offs: it’s not possible to reach the optimal state of something without the detrimental of the state of something else. 

For this reason, the body will optimize its global growth in order to make the whole organism to work correctly. Thus, all the things explained above will prevent an ant to be as big as an elephant.

Giant arthropods = a delicacy for predators

Although it seems illogical, giant arthropods would be probably more vulnerable against predators; specially, during molt. 

After molting their cuticle, arthropods go through a low activity period until they cuticle is totally hardened. In this lapse, they could be more easily predated. 

Apparently, the cuticle would imply more disadvantages than benefits for large insects!

 
Physiological constrains: the blood flow and the problem with atmospheric oxygen

Arthropods have an open circulatory system: instead of having arteries and veins to channel the blood, arthropods possess open sinus where blood bathes the organs directly. 

 
In which ways does this imply a constrain for a giant insect? While there is no active mechanism that pumps the blood throughout the body, it would be very difficult for a giant insect to oxygenate and nourish all its cells due to the gravity effect.

 Fig 5. Open circulatory system in a general insect (H. Weber, Grundriss der Insektenkunde, (1966)).

On the other side, most insects breath passively through their spiracles, which connect with an internal system of branched conducts called “trachea”. 

 
Thus, they don’t develop any active system to force air to enter inside their bodies, but it enters passively throughout these “trachea” and reaches the inner of arthropod’s body to oxygenate all cells.

 Fig 6. General scheme of a “trachea” (Images from the subject “Tree of Life”, Master’s degree in Biodiversity, University of Barcelona).

Diffusion of gases is effective over small distances but not over larger ones. So, giant insects would face serious problems to oxygenate their tissues if they reach big sizes.

 
In addition, current atmospheric concentration of oxygen (21%) wouldn’t be enough to oxygenate such a big organism with such a simple breathing mechanism.

 Fig 7. Trachea would become progressively bigger, so it would be more difficult to transport the oxygen to cells (Images from the subject “Tree of Life”, Master’s degree in Biodiversity, University of Barcelona).

It must be said that all these constrains are attenuated in aquatic ecosystems, where the cuticle’s weight and the diffusion of oxygen posed no problem for growth.

 
That explains why the world’s biggest arthropods (and other invertebrates) are mainly located in aquatic ecosystems.

GIANT PREHISTORIC INSECTS: WHY DID THEY DISAPPEAR?

According to fossil records, insects reached their biggest size 300 My ago between the late Carboniferous and the early Permian. One of the most famous examples of giant prehistoric insects is Meganeura monyi, a dragonfly that would have reached up to 70cm long.

 Fig 8. Picture of a Meganeura monyi (by Emily Willoughby)

But, the question is: Why did they disappear?

 
From 30% to 21%

Paleoclimate data shows that the concentration of atmospheric oxygen was, by then, up to 30%, so it would be oxygen enough for the maintenance of insect’s tissues. 

Some millions years later, the concentration of oxygen decreased gradually until reaching its current concentration (21%). 

It’s considered that this phenomena could be correlated with the decreasing of insects’ size.

The rising of birds

Between late Jurassic and early Cretaceous (150 My), atmospheric oxygen progressively increased, but the fossil record shows no evidence of any increasing tendency in insects’ size; instead, they grew smaller. How could we explain this?

According to a more recent hypothesis, this period coincided with the appearance of the first birds, which probably fed on big insects. So, it’s suggested that it took place a positive selection in favour of small insects. 

But not only birds: bats and also other predators could had contributed to the selection of insects’ size. However, the lack of a complete fossil record makes it difficult to confirm this hypothesis.

The most probable scenario is that a combination of ecological and environmental factors would have determined the body size of insects.

https://allyouneedisbiology.wordpress.com/tag/insects-growing-in-high-oxygen-environment/