Things people know: why are insects small?

An anonymous questioner wonders: Do ants and cockroaches have blood? And why are they so small? Two questions that are one: the insects are small because they have no blood.

As everyone knows that the body fluids of the insect are not red, insects do not have hemoglobin which is the substance that colors our blood (and the blood of our brothers in the vertebrate system) red. Hemoglobin is an iron-containing molecule whose function is to bind oxygen in the lungs and release it to cells in all parts of the body. Insects do not have such oxygen binding molecules and they do not have lungs. Hollow tubes called tracheas extend into the body through openings in the hard covering of the insect's body. Atmospheric oxygen permeates through a passive process - diffusion into the tracheas and from the air in the tracheas to the body cells.  

The passage of oxygen in diffusion is a very slow process based on the random movement of molecules. Anyone who has tried to sweeten a cup of coffee by adding sugar but without stirring - while waiting for the sugar particles to reach all parts of the cup by virtue of their random movement knows that the coffee will cool long before the sweet taste is felt. Because this is how no cell of the insect can function if it is too far from the air spaces. An insect that grows to the dimensions favored by horror movie producers will simply suffocate. Therefore any animal that is more than two centimeters thick must actively transport the oxygen to the cells. We do this by binding the oxygen we breathed into the lungs to iron atoms in hemoglobin.

There are crabs, spiders and other invertebrates that manage to grow to impressive dimensions by binding oxygen to a copper atom in a molecule called hemocyanin. The color of oxidized iron is red i.e. our blood is red for the same reason that scrap metal and the planet Mars are red. Oxidized copper is green or blue in color (as evidenced by the world famous piece of copper - the Statue of Liberty in New York). There are many large crustaceans with gloriously blue blood, but the insects prefer to settle for colorless lymph fluid.

The insects reached their peak size during the coal period (about 300 million years ago) when the concentration of oxygen in the atmosphere reached 35% compared to 21% today. Those glorious days left us fossilized insects whose wingspans reached 70 cm and dragonflies the size of crows hovered over the puddles. The oxygen concentration decreased since then and reached up to 15% about 150 million years ago and with it the insects shrank. The increase in oxygen concentration since then has not been accompanied by an increase in the size of insects and evolution researchers explain this with the appearance of flying predators (birds followed by bats) which caused natural selection to favor small and agile insects.

How is it that a million and more species of insects over tens of millions of years of evolution did not develop an active oxygen transmission mechanism that would allow them to grow? The puzzlement grows even more when you consider that not only distant relatives of the insects such as crabs and spiders use hemocyanin to transport oxygen, but there is at least one insect that has hemocyanin in its body fluids. This insect belongs precisely to the most primitive group of insects - Plecoptera. The insects "chose" during evolution to abandon the active conduction of oxygen and pay for it with dwarfism. It turns out that the insects not only can't grow, but it's also not particularly useful for them to grow.

There is nowhere to grow

The first reason for preferring the compact dimensions of the creepers is that they have nowhere to grow. Unlike us, who have internal bones that simply grow into the space around us, the insects wear an exoskeleton (the one that creates the characteristic cracking sound when you step on the cockroach). An exoskeleton has important mechanical advantages - it is the most efficient way to extract strength from a given mass of rigid material. In order to obtain, using an internal bone, the same strength that an animal produces from an external shell that is about a tenth of the diameter of the leg, we will need a bone that takes up almost 90% of the diameter so that there is not much room left for the muscles. Legs whose skeleton wraps them from the outside (insects, crabs, spiders and their friends) and about 130,000 other types of molluscs that, as oil indicates, also have no bones. Only about 60,000 species of vertebrates (fish, amphibians, reptiles, birds and mammals) have an internal skeleton. An exoskeleton also has limitations, in order to grow the animal has to discard the small skeleton and then build a wider skeleton in its place. In the interim period, between the shedding of the old skeleton and the hardening of the new skeleton, the muscles alone have to hold the weight of the body and this is a load that becomes heavier as the insect grows. For aquatic invertebrates, gravity does not pose a problem and indeed crabs, lobsters and other marine arthropods sometimes grow to impressive sizes.

There is no time to grow up

Cope rule (Cope's rule) states that the body dimensions of animals tend to grow over time: the elephants, the horses, the whales and we too are bigger than the ancestors. The explanation is simple: large individuals cope better with predators and competitors and therefore produce more offspring. Even with large adult insects, they are more successful, but the Kop rule does not work: the body dimensions remain constant throughout the generations. The solution to the contradiction lies as a mirror in the life cycle. The insects found an ingenious solution to meet the two tasks that every living being must fulfill - to eat and reproduce. Unlike vertebrates whose bodies have to handle both tasks at the same time, insects have a life stage that is specifically adapted to eating and growth and another stage in which their bodies are optimally adapted to reproduction. The eating stage is the larval stage - anyone who has raised "silkworms" (which of course are not worms but larvae) in a shoebox knows that the larvae are eating machines and do very little except gnawing. When the insect has eaten enough, it changes the shape of its body (usually in an inactive phase it is the pupa) to the adult form that deals with reproduction - finding a partner and laying eggs. A large adult individual has an advantage over its smaller competitors but a large larva has no advantage. A large insect needs to eat more, which means to extend the duration of the larval stage (in which it is more vulnerable to predation) and to extend the time until sexual maturity, which means to have fewer cycles of offspring.  

It is forbidden to bring excess weight on the flight

Most insects at some stage in their lives fly, in order for an object to fly, a lifting force must act on it to balance the gravitational force that pulls us all towards the center of the earth. The lifting force is relative to the speed and the minimum speed for stable flight (stall speed) is relative to the size. Large bodies have to move faster to avoid falling. The insects manage to fly slowly, without investing much energy because they are lightweight. Even among vertebrates, the flying ones tend to be small: bats are light and smaller than most mammals and flying birds are smaller than those that have lost the ability to fly (such as the ostrich).

Insects are the most common animals around us and yet they represent a different world: an evolutionary branch that chose opposite solutions to ours for the challenges of existence. Since the monstrous grasshopper in the prophecy of Joel, whose "teeth were the teeth of a lion and caterpillars brought him" until Horror Films Insects and spiders invading our dimensional world are a powerful means of instilling fear andDisgust. Fortunately, that same biology ensures that such creatures will remain in the realm of fantasy.

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More of the topic in Hayadan:

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