EVOLUTION: INSECT SIZE

INSECT EVOLUTION, SIZE LIMITATION CAUSES

There may be several reasons insects are all rather small.  But the principle reason limiting size of insects is the design of their respiratory system.  A clue to this may be found in comparison of insects and crustaceans.
Crustaceans also have an exoskeleton that must be shed for growth to occur, but they can be many times larger than any insect.  There are no truly marine species of insects beyond shoreline regions.  But large crustaceans are found in the ocean, in freshwater, and upon land.  So molting and the support provided by living in water are not the answer.

Both have blood in the body cavity. Blood enters the heart and is pumped out through arteries, bathing the organs in the body cavity as it returns to be pumped out again.  In the crustaceans the blood entering gills or flattened appendages is oxygenated as carbon dioxide is discharged.  That respiratory exchange takes place in the interior of the insect as the blood passes around the smaller tubules of the tracheal system.

The tracheal system is a system of tubules with openings on the surface of the insect.  The interior surface of the tracheal tubes and tubules is continuous with the outer chitinous exoskeleton layer covering the insect.  The tubules are closed on the end inside the insect.  The finer terminations may be water filled when the insect is inactive.  Increased numbers of solute molecules due to metabolic activity can cause water to leave the tubules and assist movement of oxygen into the blood.  The highly branched system's fine terminations in all parts of the body substitute for the need for a capillary network of a circulatory system.

The main factor for size limitation

Two facts may be responsible for limiting the insect size due to tracheal system function.  1- large size means greater hydrostatic pressure on the tubules may collapse them and inhibit respiration.  2 - large size means greater length of tubules may slow diffusion of oxygen into the regions more distant from the spiracle on the exterior surface.

The limitations of the circulatory and respiratory system and the resultant small size may also be a factor in preventing warm-blooded insects from developing.  Some larger insect such as certain bees and moths have larger bodies and ability to sustain a warmer temperature than the environment by "furry" bodies and a high rate of activity.

It is of interest that even aquatic insects have air-filled tracheal systems.  Several different mechanisms are used in different aquatic insects to oxygenate those tracheal systems.

It is likely that a crustacean ancestor, perhaps now extinct, gave rise to insects.

Small organisms have the advantages- of finding more hiding places, needing less food per individual, reaching adult size more rapidly, higher reproductive potential, and as is evident around us - having more species and specializations exist in a small area.

Joseph G. Engemann     Kalamazoo, Michigan   October 8, 2015